What is a gas blow-back gun...?

I'm looking in to buying some BB guns just for fun with friends and stuff but I want to make sure I'm getting the best so we're doing our homework. I'm doing pistols (although I assume the information can be used for them all should any of my friends fall short). Anyway, I got to the pistols page and it asked me to choose between "springer", "AEP" or "GBB" guns but quite frankly I didn't have a clue what it meant. So after some research I found that springer just meant a basic gun that must be manually ****** every time a bullet is shot and AEP was basically the same but cocking is done electrically but couldn't find decent information about basic GBB guns. So, to your gun enthusiasts I ask: what is it all about, and what are the main advantages and disadvantages between all these categories? Which should I get? Easy ten points, help soon! Thank you.