It's not just you, I figure that most would be a bit suspicious. Or curious. Who submits a form with multiple spoilers? That's what I'd think.

Should I start trying to play vague mind games whenever I talk about Alethea too? As to mix up what's exactly hidden?

I don't know I don't know?

neither, but I think both.

Hi Time! Know what time it is? It’s time for…your very very overdue F/GO gameplay explanation! 

First off, I gotta explain some of the lore. Very basic stuff, so don’t sweat it.
The character you’d be playing as is a Master, the person who summons Heroic Spirits and forms contracts with them. Those contracted Heroic Spirits are called “Servants.” And for the purpose of this explanation, I will only be focusing on mechanics that relate to Servants because, frankly, it isn’t really necessary to understand the ones that pertain to the Master character.
In-lore, Servants/Heroic Spirits are basically ghosts who are much, much stronger than the average human. Those with the weakest stats–for instance, the weakest strength–are still able to snap normal humans like twigs. Their strength comes from their fame, in being known by the masses throughout time. And so accordingly, Servants all have something called a Noble Phantasm (abbreviated as NP for short)–whether it’s a mystical weapon, technique, or culmination of their stories–that they can use as their ace. For the purpose of F/GO, we will usually be talking about Noble Phantasms that need to have their names spoken aloud (those names are called True Names) in order for the NPs to be used.
Outside of Noble Phantasms, we have things called Skills. And those are separated into Active and Passive Skills. Gameplay-wise, Active Skills are the ones you click to trigger, and passive ones are gameplay boosts that are always active. 

Typically, Passive Skills are Class Skills, which brings us to what Classes are. Each Servant is summoned under a Class, and the typical 7 are: Saber, Archer, and Lancer (the Knight Classes), and then Rider, Assassin, Caster, and Berserker (the Cavalry classes). Other classes include Ruler, Avenger, Moon Cancer, Foreigner, and Alter Ego. There are certain Servant classes that are not included in that list, but those are the ones you will see most often. As for what the class skills actually are, basically each class usually has one or two passive skills inherent to it that are assigned to a Servant when their Spirit Origin is designated as said class–for example, Riding and Magic Resistance for Saber, Independent Action and Magic Resistance for Archer, Presence Concealment for Assassin, Mad Enhancement for Berserker, etc.--but that doesn’t factor too much into the overall gameplay aspect, so I won’t go too into depth here.
Each effect has its own small gameplay effect depending on rank. For instance, Magic Resistance marginally increases debuff resistance, and Mad Enhancement increases a certain card’s performance. More on card types later.  

In F/GO, different classes have different advantages over each other, called Class Advantage/Disadvantage, though this particular concept does not technically manifest into lore because it doesn’t logically make sense in some (many?) cases. (That sentence is the bulk of the idea; you don’t necessarily have to know them, so feel free to skim the rest of this paragraph if you’d like.) The Knight Classes have their little trinity of Saber over Lancer, Lancer over Archer, and Archer over Saber. Similarly for Cavalry, we have Rider over Caster, Caster over Assassin, and Assassin over Rider. Berserker generally has Class Advantage over every class except for Foreigner. Oh yeah, and by “Class Advantage,” that means certain classes deal more than 1.0 damage to others, or take less than 1.0 damage to each other. Contrary to that, class disadvantage means the opposite. But notably, a class doesn’t always have both offensive and defensive advantage over another class. For example, Alter Egos have offensive advantage over Cavalry classes but take neutral damage, and Ruler classes have a defensive advantage over most classes (exempting Berserker and Avenger) but deal neutral damage to all classes except Berserker and Moon Cancer, which they have offensive advantage over.
Somewhat related to Advantage, classes like Assassin and Caster typically do not excel in combat, so their base attack stat is pinched in-gameplay. For instance, a Caster could have an attack of around 10000, but their actual base attack numbers while in-battle will be around 9000. So that can suck, I suppose. 

That’s most of the lore! Now onto gameplay for real.
F/GO gameplay is known for being super simple. You kinda just. Click buttons. Then watch things die. (Or watch your own characters die, depending on the quest difficulty and your level of preparation. [: ) The game operates by turns, meaning you can just not click buttons and vibe to the music to mock the enemy. 
However, if you do want to click buttons, you first have skills to activate. If a Servant is fully leveled, they will have 3 active skills, which operate on varying cooldowns. And since 3 active Servants are able to be on the field at a time, you have 9 skills at your disposal. Skills serve to buff your Servants or to debuff the enemy (a single one or all of them on screen at the moment), which varies depending on the Heroic Spirit. But as mentioned before, they have cooldowns, so you need to time them correctly. Typically, Servants will either buff the entire team on-screen or a single Servant (themselves, or a selectable target). 
Then, after the skills comes card selection. Each Servant has a deck of 5 cards, and so every three rounds, the on-screen Servants’ cards are pooled together and shuffled to give you a choice of which ones to pick. And for these cards, there are three types: Arts, Quick, and Buster.
Buster is associated with big damage. It has a higher damage output than Arts and Quick. 
Arts is associated with NP gain. Did I mention that the goal for most Servants is to get their NP gauge up to at least 100% so that they can use it? Well, Arts cards are typically associated with that end goal.
And last but not least, Quick is associated with critical stars. You need 50 to be able to know for sure that you’ll have 100% crit chance on all of your Servants’ cards. But usually you’ll have to play the game of hoping cards will crit instead. Of note is the fact that different classes and Servants have different chances of gathering crit stars to their cards and thus being able to crit better, with Berserkers and Avengers particularly being notorious for not being good at gathering stars. 
Also notably, Quick is not seen as favorably by the fanbase because its damage is worse than both Arts and Buster, and extra crit stars currently have no value. But it is also able to generate NP towards the NP gauge. It’s just that some Servants do it better than others. 
The order in which you put your cards also does matter. If you start a chain off with Buster, for instance, the entire chain will do more damage. If you start a chain off with Quick, more crit stars will be generated (though the actual impact will depend on the hitcounts of the cards as a whole). And if you start a chain off with Arts, more NP will be generated. Also, the second and third cards will have a slight boost to their effect (slightly greater damage up for Buster, slightly greater NP gain for Arts, and slightly greater crit star generation for Quick). So that should also be considered.
Notably, the chain effects mentioned above do not apply to Noble Phantasms, which are their own cards and will be discussed mechanic-wise in a bit. Basically, a Noble Phantasm will generate the same amount of NP/crit stars and do the same damage no matter what its card placement is. A bummer to be sure, but it is what it is. 
If you’re able to select 3 of the same Servant’s cards in a turn, you will gain a fourth card automatically, which does extra damage, generates crit stars, *and* generates NP! This attack is called the Extra attack and is very helpful. You can use both normal cards and the NP card to create a Brave chain, which is what this chain is called lol.  As promised, the Noble Phantasm card of a Servant appears when their NP gauge is at least 100%. It is also designated as one of the card types (Arts, Quick, and Buster), and generally speaking, there are two roles/designations of an NP: Damaging and supporting.
Damaging NPs can be AOE (area-of-effect) and hit all the enemies on the screen currently, or single-target (and hit the individual enemy you have selected). Supportive NPs, meanwhile, can target the entire team (including self or not), oneself, or (in one particular case that I can think of) a single team member to help it offensively or defensively. Whether an NP is damaging or supporting depends on what it actually is and what it’s sourced from. For instance, King Arthur’s NP is Excalibur, a very high-rank NP (btw most everything is ranked lol, from Servant rarity to NP rankings and skill rankings), and basically she shoots a beam with it. So it’s an AOE Buster NP. One of Echo’s favorites, Paris of Troy, has a single-target Quick NP that first applies Ignore Invincible (a hard defense buff in the game) to him before dealing damage. Specter’s favorite, Waver, has an Arts NP that debuffs the entire enemy team depending on chance. And my favorite guy, the Prince of Lanling, has an Arts NP that marginally debuffs the enemy and buffs the entire team. So it really depends on the Servant’s lore and personality, the latter being in my view.  
By the way, NP levels scale up to five. You start off with one when you roll one copy of a character, and get one NP level each additional copy you obtain and use to level up. Level one NPs max out at 100% gauge, while level 2-4 max out at 200%, and level 5 maxes out at 300% without Overcharge. 
Speaking of which, Overcharge scales up the damage of damaging NPs and increases the effects associated with any Noble Phantasm. It can be achieved by following a Noble Phantasm after another (resulting in an NP chain effect) or by having an active Overcharge effect on the Servant in question. This effect is more helpful for some Servants than it is for others, but usually it’s quite beneficial.  

Last but not least, I think it’s important to note that in this game, damage stacks multiplicatively rather than just being added together. The three buff types that multiply together for Noble Phantasms are: Attack up (with a little sword icon), card type up (Quick, Arts, or Buster up), and NP damage up (NP strength). The NP strength buff obviously does not apply for Servants with non-damaging NPs, but teambuilding-wise, it’s important to remember this fact to do the most optimal (highest) amount of damage. Otherwise, teams vary from stalling teams (defense up and healing oriented) to crit teams where you may not even use your NP cards. It depends on the playstyle you’d be going for. 

And headcanons-wise, I think all I can add is that different card types kinda have different vibes (subjectively speaking). Buster, as a more common/the easiest example, is associated with damage, so it’s associated with characters who fight a lot, are aggressive, and/or super powerful. For example, King Arthur, Hercules, and Attila the Hun. So similarly, card decks have different vibes they give off as well, at least in my opinion. As a prime example, Berserkers tend to have triple Buster deck cards, meaning it’s harder for them to generate NP/crit stars but they will dish out a bunch more damage if you manage to get a Buster-Buster-Buster Brave Chain off. Meanwhile, Casters can tend to have triple Arts decks, and Assassins triple Quick. But this obviously depends on the character. Most characters have 2 cards of 2 different card types, and then a single card for the remaining card type. Namely: BAAQQ, BBAAQ, BBAQQ. Added to their NP card type, of course. 

But yeah, that’s about it! I’m not too sure how comprehensive this is, but I do hope it’s something at least. I decided not to get into the different buff and debuff types and such because that would just be so much more unbearably long, but do let me know if you have any questions or any character you’d like to know about or something. I dunno why you would want to go through me as the explainer but xD. Just getting it out there. 
Sorry for the super long wait, I knew it’d be easy to get out so have been focusing on harder things lol.
Happy travels and let me know if I should have asked you instead of just sending this whole thing over! (‘:

-Galaxian-

(':
Actually, the answer for that is supposed to be pretty simple! The answer is that there's a certain quality that a Heroic Spirit will have that will qualify them for a class. For Saber, it's the use of a sword/saber, or similar weapon. For instance, King Arthur's use of Caliburn and Excalibur. 
But this is joked about a lot in the fandom because sometimes there doesn't seem to be any reason for a Servant to qualify for a class. For example, there's a notorious quote called 'The Archer class is really made up of Archers!" which will usually be used to refer to an Archer who is not using a bow. For example, Ashwatthama from Indian myths uses a giant burning wheel to attack, and he's an Archer. Chiron the Centaur can use a bow but mostly will punch you. Jeanne d'Arc (her Summer version) will yeet dolphins at you. But myth-wise, Archers use projectile weapons (throwing weapons at you counts as projectile), and typically have a very useful/good NPs.
As for the other classes, I'll just briefly go over their qualifying attributes:
-Lancers: Use of a spear/polearm weapon
-Rider: Typically the possession of multiple NPs, history of riding a mount
-Assassin: ...assassination.
-Caster: Powerful magecraft or magic usage
-Berserker: History of having gone insane in their lives in at least one occasion
-Ruler: Lack of a Grail-related wish (as an arbiter) or a Saint
-Avenger: Hated by humanity 
-Moon Cancer: This one is weird but I think it could be summarized by a being that exists outside of world norms (it's supposed to be a class for a single character but other ones keep getting shoehorned in in F/GO lol)
-Foreigner: Being who exists outside of Earth; examples include beings affiliated with Eldritch gods and a cute space probe (Voyager 1)
-Alter Ego: Culminations of a single aspect of a character that now has its own form and way of existence

-Galaxian-

I dunno man, the ratio feels like it's 50/50 at best. xD
In lore, Servants who lived closer to the present time are less powerful and thus are less likely to be summoned. But you are very correct. Examples include:
-Thomas Edison (though his case is weird and you should definitely see what his design looks like) is a Caster, though Nikola Tesla (who is specified to be very powerful because of what he did) is an Archer rather than a Caster. A
-Paracelsus von Hohenheim, a philosopher and alchemist, is also a mage in the Fate verse.
-Writers, artists, and musicians such as Shakespeare, Hans Christian Andersen, Leonardo da Vinci, and Amadeus Mozart are also Casters.
-Tacticians like Zhuge Liang and Chen Gong are Casters too.
So basically you are super right! :D But these Casters tend to suck at combat. So, in actual works, Casters tend to be more combat-oriented. Not in F/GO though, the cast variety is big in this one.

I'm assuming you were saying "this character" as in "Moon Cancer"? I am pretty sure that's right. In fact, most of the Extra Classes (outside of Knight and Cavalry) were made for plot purposes but then were incorporated in-gameplay for more characters. If you see a new class being introduced, that's almost for sure a sign of an antagonist. But basically you're right again. xD

-Galaxian-

xD it do be like that at early morning hours.

Casters? They try not to die. And do a lot of scheming. Ironically, it tends to work pretty well. I don't think I can remember a single Caster who truly died with regrets during a Holy Grail War. It's only afterwards, when their crimes against humanity catch up to them.

The creation of a new class can be expected for major derivative works. At least I think. So basically new classes can be expected to pop up in new Fate works lol. 

-Galaxian-

I don't think it could be phrased like that. I think the Moon Cancer class specifically is very world-based. From my understanding, they're an extremely big threat when they're on the Moon (which is where the derivative work the class is sourced from is set, so the class is probably the Big Bad there, I'm guessing). 
It's very hard to call a class meta or not. It really depends on the battle and mechanics involved. But thinking about it, I don't think I've ever heard anyone call the Servants in the class the elite of the elite, even with a recent addition in mind. So that's kind of funny.

Fate works come out pretty often from what I can think of. Just this year there's been the game Fate: Samurai Remnant and Fate: Lost Einherjar, if there haven't been others. What I've observed is that the plots tend to focus on "Holy Grail War but with a twist" (basically not the traditional 7-Servant War held in Fuyuki, Japan). For instance, Apocrypha has 2 teams of 7, Strange Fake has "fake" and "true" Servants for each class (if not more tbh I'm not sure), etc. But there are exceptions too, like how a spinoff called Case Files mostly explores the magecraft/magic side of things. All that's to say that the franchise is pretty big and there's a starting point (namely, one or two routes of Fate/stay night and Fate/Zero) that I would recommend someone go to first before exploring the rest of the series.

-Galaxian-

All sorts of people. Mostly Japanese. (if not all)
In more seriousness, I think it's mostly fans of the franchise? They get around to making their own series. It's pretty neat, it's like fanfiction made canon in some multiverse out there. Though obviously characters can be kind of restricted because they're usually actual people or original characters of the past (legendary or historical figures), and so actual original characters are limited. Oh, and now seems like a good place to mention that when I say "derivative work," I mean pretty much every work other than Stay Night, and maybe Zero. Since they all originate from that one (or those two depending on your perspective). 

Idk, I feel like they can't just be tiered like that. In F/GO there's really a difference depending on situation. For example, Challenge Quests, story, farming, gamestyle, in general (everything combined in consideration but this tends to be more controversial). Also it depends on the rarity (and if you've grailed a Servant), since their stats will then be different.  

-Galaxian-

Lol, that's fine. It's fine even if you will forget to respond in the future. 
I think there's probably a process for approval, like creators probably have to pitch their idea to some committee or something first. Otherwise, I imagine it'd be hard coordinating the marketing and canonicity, and conflicts relating to copyright would probably arise. Essentially, there's probably a process to differentiate these works from fanworks. 
There aren't really game modes, but for the developed account it typically comes down to story, farming, and challenge quests (high difficulty nodes). In F/GO, versatile kinda translates to OP tbh. 
I'm realizing the way I am talking is probably not portraying the game in a good light xD oopsie.

-Galaxian-

I'm back as well (somewhat) (yes this is the thread I realized I forgot to respond to)

I'm not too sure what you mean by that, sorry. But I'll say it's possible I'm not very familiar with the most popular fanworks in general. From what I can tell, though, what I said before was true. Japanese fanworks are more likely to get accepted because, well, it's the same language, it's easier for the higher-ups to vet the work and communicate with the creator. 

The saying "Jack of all trades, master of none" begs to differ xD (just kidding here, by the way.)

-Galaxian-

Nah I was the one who nearly left you hangin', sorry about that. xD
Not the main entries (because they were the ones that established canonicity and started the series off after all), but after searching around a bit, I think a few spinoffs became canon while starting as fanworks. For instance, an ongoing work named Fate/strange fake started off as an April Fool's joke, and I think I recently heard that a French comic artist is starting to collaborate with TYPE-MOON (which is the company responsible for coordinating the franchise, I believe). Hence why in Fate there's a joke called, "Jokes are truly the deepest lore," which I think I've referenced in the past.
Unfortunately, it seems that the answer is ultimately no then, since for Fate it's hard to define what a "main entry" is. For all I know, the sole main entry for the series is Fate/stay night, and it was truly the original before the work became a series of any sort, so it had to establish the popularity that would cause what we're talking about to happen, if that makes sense. 
Have you seen a series/franchise where this pertains? The closest I can think of at the moment is Saint Seiya, where everything is supposedly happening in a multiverse somewhere. But canonicity still remains a problem, and that itself does a fantastic job of dividing fans for sure.

-Galaxian-

You say as I nearly forgot this again (the post history thing needs to stop updating so quickly xD)

Yeah. That's kinda sad.
I think SS really shows the duality of letting a fanwork have its own plot, because the author is apparently infamous for breaking the plot points of spinoffs that tried to remedy his plot holes from the OG series. For instance, there was an author who picked up the prequel series that the original author was making, named Next Dimension, and named her spinoff Lost Canvas (abbreviated LC)...but then the original author (Masami Kurumada) made everything in LC non-canon by continuing his prequel work anyways, and that divides the fandom because some people like one over the other and have strong opinions on it. It's kinda funny tbh xD.

-Galaxian-

Lmiao an author okays a fanfic and then the writer starts doing just the most cracked crackships
Y'know, the funny thing is I think I forgot how fanfics actually are, since for me I tend to focus exclusively on elaborating on canon events and relationships, and in SS particularly there's more so the power contest (between generations or other works in the series) rather than shipping of any sort xD. But anyways, I think it's actually not that hard for fanworks to be moderated. So far it seems the bigger problem is canon fanworks not being able to go to their greatest potential because of said moderation. What do you think, do you have any examples of the opposite?

-Galaxian-

I mean, do you have any examples that you know of where a spinoff is not moderated enough and things go south. (Because I don't xD)

-Galaxian-