The Fighters Guild
Expect this guild to be much like it was in Morrowind. It will focus on developing the combat skills of members and will likely offer quests involving the killing of certain NPC’s or creatures. The GameInformer article describes it as one of the two “noble guilds”.
The Mages Guild
The other “noble” guild, this guild will focus on the magical arts. If Morrowind is any indication you should expect numerous quests to gather alchemical ingredients or to retrieve lost artefacts or books.
The Thieves Guild
The last of the factions that appeared in Morrowind, you should expect numerous missions to…say it ain’t so…. steal stuff! This should actually be much more interresting then it was in Morrowind as inside sources (including one developer who used to work on Thief) say that thievery will be a much bigger part of the game then in Morrowind.
The Dark Brotherhood
This assassin’s guild made an appearance in tribunal but is joinable in Oblivion. It should be much more sinister then the relative nobility of the Morag Tong from Morrowind. Being an assassin will probably also be much more interresting to the improvements on the combat and thieving systems.
The Nine Divines
A variant of the imperial cult from Morrowind, this is the only religious guild that has been announced yet. Apparently it is the guild to join if you want to be a monk.
This is the guild to join for gladiatorial combat. I would expect that it would involve beating single opponents one on one as in the duels that were fought (rarely) in the arena in Vivec.