So are the factions international or is it a seperate one for each province? Like are the Cyrodiilic Mage's Guild and the Morrowindish Mages Guild the same, just different branches? Same with the other guilds? (Except for thieves guild, I assume)

(And yes I know the Mages Guild no longer exists, it was just an example )

I'm pretty sure they're still chartered by the Empire. As I recall, in the Morrowind Mages Guild, if you did all of Skink's quests, he would give you a letter from Ocato to deliver to Trebonius Artorius telling him to step down as acting leader of the Morrowind branch. And promote you to Arch-Mage. So there is someone higher up than him back in Cyrodiil.

But yes, I think each province has its own guild branch, each with its own local head. Even if they answered to the Empire.

What also should be pointed out is the Morrowind Fighters Guild Master, Sjoring Hard-Heart, apparently was convinced that the Empire was going to collapse in the near future and as a result made an alliance with the Camonna Tong to ensure the Morrowind branch would survive as a seperate entity.

Imperial factions, like the Mages Guild, are Imperial organizations and therefore technically the same throughout the provinces. However, the actual operation of them varies greatly to appease local customs. Guilds like the thieves and dark brotherhood are independent however.

well...I wouldn't say independent as the correct term with the TG and DB, more like decentralized. There is still the overarching guild, but there really isn't a big home base, boss, or imperial charter.

That may be true of the DB (though I'd say that their operation in Morrowind, Cyrodiil, and the Iliac bay varies enough to declare them wholly autonomous), but the thieves guilds are http://imperial-library.info/content/oblivion-myth-or-menace

About that book, I dunno, but I can't really put too much faith in it. The Cyrodiil branch may be a lot more secretive, while the MW's branch was, more or less, an open book, but went secret when things went south (or was destroyed). No mention of DF's chapter either.

But what makes me feel that they're not unrelated is they all share the same name, Dark Brotherhood and Thieves Guild, and they pretty much do the same job, but with some variation. Example, how the DB operates and is like does seem to depend on the province they're in, but it's still unified in that they will not target each other (save for traitors), have similar initiation and recruitment rites, and kill for money. Same deal with the TG, for the most part.

They just seem like very decentralized organizations, instead of being wholly independent from each other. What happens in province X will not affect province Y, but there is still the same guild should a member relocate from X to Y, and operate there.

Something gave me the impression that the book "Myth or Menace?" wasn't an entirely reliable one, anyways.

Theres really not much variation you can achieve in the quests (DB: kill people for money, TG: steal stuff), so you have to look at the specifics of the operation. Killing guild members is just bad for business. I also wouldn't place much importance on a name, especially one as generic as "thieves' guild." Lets break it down:

Leadership:
Oblivion: Gray Fox, one identity assumed by a succession of people.
Morrowind: Gentleman Jim Stacy.
Daggerfall: n/a
In all three games no united leadership or inter-province cooperation is mentioned. Morrowind even has a rival 'thieves guild,' the Camonna Tong, and used to have a the Bal Molagmer, a guild of thieves who helped the poor.

Joining:
Oblivion: get out of jail, beat other thieves in a test of skill.
Morrowind: ask.
Daggerfall: successfully pickpocket 10 someones or break into someone's house.
While Oblivion values a criminal background, Daggerfall prefers not being caught by the guards. Morrowind can be disregarded, since none of its factions have prerequisite actions.

Quests:
Oblivion: Focuses primarily on a war with the Imperial Legion as well as general helping poor people. High level quests are all about getting artifacts for the Gray Fox.
Morrowind: Mostly miscellaneous thievery, high level quests focus on the conflict between the thieves, the Camonna Tong, and the Fighter's Guild. Optional help-the-people quests inspired by the Bal Molagmer.
Daggerfall: Stealing random stuff.
The Morrowind Thieves guild is much more of a criminal organization than that in Oblivion, though we cannot know how it was prior to the Umbranox being the Fox.

Halls:
Oblivion: none. Quest givers (all 2 of them) spend most of their time in their abodes. The Gray fox appears at other people's houses. The fences hang out at their homes or wander outside, though a couple do spend significant chunks of time within taverns.
Morrowind: Taverns, stores, and other establishments.
Daggerfall: proper guild hall.
Its obvious that the guild is tolerated better in Daggerfall than in either Morrowind or Oblivion. The Camonna Tong, the Dunmer's thieve's guild, does have its own hall.

When compared to the Imperial factions, which have no joining requirements in any of the games (besides having your skills above a certain number), and fairly consistent leadership arrangements and guild halls, as well as the existence of multiple thieve's guilds in Morrowind, I'd say that its pretty certain that the guilds are not affiliated with each other.

The Dark Brotherhood is a different matter, given that they do share a specific name and have a lore-backed origin. I will agree with you that the DB likely is one decentralized guild.

The thing about the Camonna Tong's guildhall is that it isn't explicitly called "Camonna Tong Guild Hall" or anything. It's a corner club. They remind me of an Italian or asian mafia syndicate in that regard. In fact, the term http://en.wikipedia.org/wiki/Tong_%28organization%29 is likely based on secret brotherhoods that originated in China and continue to exist in Chinese neighborhoods worldwide. Many of the same members frequent the same bars, social clubs and other public places but it isn't like there's a big neon sign saying "the criminal organization meets here." It's more like an open secret. They're tolerated and most people know they exist, but at the same time there's sort of a glossy veneer of these places being respectable businesses. The fact that the Camonna Tong is the native syndicate to Morrowind and the Thieves Guild are viewed as a foreign influence probably means more Dunmer sympathize with them and their goals, and there are probably many instances where local guardsmen look the other way. In fact, rumors from Oblivion suggest that the Cammona Tong eventually succeeded in destroying the Thieves Guild on Morrowind.