Nothing, In game doesn't really tell you anything about them and I'm not going to go searching for grimoire cards to find out. So I just join a faction for gear, weapons, shaders and the exotic class items, nothing else about the factions really matter. I have run through Dead Orbit and New Monarchy and am currently working my way through FWC. I don't really play much anymore though, so I might never get the exotic class item from FWC.