DoubleSb

Electric runs off of a battery that you charge and is stored in the gun, usually in the grip or stock. The battery powers a gearbox and springs that powers the bolt which hits the BB and sends it flying down the barrel.

Gas is a lot more uncommon and is usually only on pistols, you but a little co2 cartrige in the grip and with each pull of the trigger, a gust of co2 sends the BB flying down the barrel.

Batteries are a lot cheaper and last longer. Pistols can have batteries too.