There isn't much info on them. We've discussed them on here before, also search the archives of alt.games.daggerfall (usenet) for info, there is a lot of forgotten information there.
Inside there are a bunch of NPCs (from memory about 5) associated with the underworld, including the "unique" NPC The Crow who is a high ranking member of the Thieves' Guild.
The Crow doesn't give Thieves' Guild quests, I think he offers merchant quests.
They don't sell anything. That's about all there is to them.