How do air soft guns work?

I have several for some reason. Some work on a gas cylinder, others are electrical and need to be charged up.

Typical airsoft guns do not use gas or batteries (the good ones run on batteries, though), but instead propel the small plastic pellet via air thrust.

This is for an Automatic, electric airsoft gun, more commonly known, as an AEG. This won't be very in depth- just a general overview.

When you pull the trigger, some electrical contacts are brought together, completing the circuit, containing the battery and motor. The motor will turn (Or the cog attached to it will). This will turn a set of gear, the top one, has teeth on only one side, when this rotates, the teeth will pull back a piston, this piston is now being pushed against a spring. When the gear turns some more, it will come to the side with no teeth, so the piston will be free to fly forwards, with allot of force, because of the spring. This Piston will force air through a cylinder, and out a small nozzle. This air propels the BB out of the hopup, and towards the target.
The Hopup, is the device which the BB sits in before it is fired, and is also used to give the BB backspin. increasing it's range.