So far we know that the will be the equivalent to the Mages Guild, The Fighters Guild, The Thieves Guild and The Dark Brotherhood which is pretty much the same as oblivion. I also know you can join the Imperial Legion but as a faction not as a guild (not sure what the differences are), and every city has its own faction, whether they are all join able i don't know. Does anyone else have more info about guilds/factions? Will we be able to join the great houses (or something) from morrowind.

they have all been taken out. No more guilds or factions, even though there are coutless threads and youtube videos that have highlighted all there is to know about it, Todd has decided to take them all out.

Thieves Guild (Joinable)
College of Winterhold (Joinable)
Companions (Joinable)
Dark Brotherhood (Joinable)
Riverwood Tavern Warrior Clan
Stormcloaks
The Shield-Brothers
Necromancer Faction (Not Joinable)
The Imperial Legion (Joinable)

Straight from the Skyrim Info Thread, only ones confirmed so far.

Dovahkiin Club...

The stormcloaks are also joinable, unless I miss my guess. Don't quote me on that though; it just makes sense since the conflict is likely to be a central part of the main questline, and not everybody is going to want to support the empire