Over at DestinyHub thesirchelios has written an article guessing at what we might expect from the different classes and factions of Destiny. So far there appear to be three known human Guardian classes (Hunter, Warlock, Titan) and five factions (Dead Orbit, Future War Cult, New Monarchy, Osiris and Seven Seraphs).

Myself, I'm guessing that factions will be how Destiny implements clan functionality. This would have the effect not only of integrating faction warfare into solo, coop and vs multiplayer, the way it does in Eve Online, but avoid some of the logistical challenges of maintaining clan lists.