Here’s a quick writeup of the major and most of the minor factions that make up the game world. This information is considered to be generally common knowledge, so as long as your character hasn’t been living under a rock he or she will probably be at least aware of the different groups, if not actively involved with them.
I am specifically withholding information as well, which you’ll only get if you’re aligned with that faction. If you choose to align yourself with a faction, I’ll send you a more detailed bios of their information which will include stuff that the average Joe wouldn’t know about that particular group. You’re also allowed to have once been a part of a faction, but are no longer, and most variations on that idea are probably fine as well.
You can also pitch your own factions as part of your character’s backstory, and I’ll fit them into the setting if possible.