What actually transfers over from my character from Alliance to Horde?

So my friends just informed me that they are faction transferring in a couple hours to play Horde for Legion, and I guess I'll be transferring my human paladin that I've been playing since Vanilla. Will I lose anything? How does rep work? ( I have a tremendous amount of exalted reps) What about items that aren't in the toy box yet? What about quests?

I Tried looking at the official blizzard page but it seems so outdated...