I've been flashing ROMs but I don't really see the difference between them. I'm on KitKat 4.4.2 for the Nexus 4.
Well AOSP [pure android from google's kitchen] is the Android Open Source Project which is the father and the mother of all ROMS out there, it is the base android code.
AOKP is kanged [copied/stolen], its based on AOSP or derived from AOSP with some mods.
Differences are mostly UX and UI based, for example HTC Sense and Samsung TouchWiz are all just skins on top of pure android. Ok not pure android there is a lot of stuff modified by the big banners. The basic idea is this, AOSP is what google gives and AOKP is what people build with features on top of AOSP.
EDIT : XDA-Developers The above mentioned thread has a lot of info if you the patience to scroll through!