The different categories of airsoft guns are: AEG,GBB and spring guns. AEG stands for Airsoft electric gun, GBB stands for gas blowback which is more realistic than the other types because it has blowback, and when you shoot it the slide slides back like it would on a real firearm. The last category is spring guns, spring guns does not require a battery or gas. You can just load in the bb\s and you are good to go. The downside of a spring gun is that you gotta pull the slide back each time you shoot.

