Right now there are three NPC guilds in the game, which you can join - all offering a unique skill line for your character:
- The Fighters Guild is pretty much what you expect them to be. They've taken a massive contract to fight back Molag Bal's Dark Anchors and all Daedra.
- The Mages Guild is pretty much what you expect them to be as well. They're trying to preserve knowledge throughout the times of war.
- The Undaunted are new in the series. They're a group of adventurers trying to prove their worth and they're a bit like a group of posers. They want to fight the biggest, strongest, meanest creatures and win.
Unfortunately, some guilds didn't make it right into launch , but they're scheduled to be added at a later date, most likely later this year:
- The Dark Brotherhood will be added later. They're still quite the same as in the previous games and will most likely offer contracts and bounties.
- The Thieves Guild will be back as well. They didn't release any details so far, but it's quite likely that its introduction will also introduce a crime/stealing system.
Also, there are quite a few other groups in the game. Some you'll be able to join, but they won't necessarily grant you direct bonuses or skills. Warning: spoiler ahead!
The Eyes of the Queen are a network of spies working outside the Aldmeri Dominion's laws (as far as necessary) and subject only to Queen Ayrenn herself.
The Morag Tong are quite similar to the Dark Brotherhood, although they're quite focused on Morrowind.
The Shadowscales are a group of Argonians bringing the gift of Sithis to those worthy. Another group of assassins (is there some pattern? ;)).
The Beldama Wyrd are a circle of witches from Glenumbra. They're pretty much classic druids, but only women.
The Companions you've met in Skyrim appear in ESO as well, but you're not able to become one of them.
And probably many others I just forgot to mention right now... this list is definitely incomplete.