From the Tamriel Foundry: http://tamrielfoundry.com/development-faq/#npcguilds
(BTW...Tamriel Foundry usually has the most accurate up-to-date info on ESO)
- The player will be able to join one (maybe more) of the iconic guilds of the Elder Scrolls lore. In order to join and progress within these NPC organizations, the player must first prove himself worthy.
- The Fighters Guild is predominately interested in repelling the attempts of Molag Bal to dominate Tamriel. By destroying the demonic Dark Anchors that Molag Bal sends forth from his plane of Coldharbour the player can gain favor with the Fighter’s Guild. These Dark Anchors are semi-random dynamic events which require small groups of players to assemble to destroy.
- The Mages Guild works to collect and catalogue the many arcane tomes which are scattered across Tamriel. By finding and collecting these books, a player can ingratiate himself with the Mages Guild.
- The Thieves Guild will be in the game, but the means of joining this organization are not yet known.
- The Dark Brotherhood will be present in the game, but whether the player can join this organization or the means of doing so are not yet known.
I'm sure you know which one I'm most interested in if you know anything about the icon under my name