Airsoft guns are used for a variety of purposes, including military training, recreational shooting, and in sports that are similar to paintball competition. The guns are usually designed as exact replica's of real life handguns, and they can be incredibly lifelike. There are many enthusiasts around the world that collect them, and over the years several different designs have been developed.
On a fundamental level, an Airsoft gun serves one simple purpose, to send a projectile (in this case, a plastic BB) towards a target at high speed. Manufacturers have found several different ways to achieve Airsoft gun propulsion, and they each have their ups and downs.
Some Airsoft guns are propelled by simple springs. These work surprisingly well, and they're very durable. These are a common choice for beginners. The disadvantage is that you have to cock the spring each time you fire, and they don't fire as far as some other propulsion systems, but these guns are very inexpensive and they're one of the most popular types.
Gas airsoft guns are a bit more expensive, but many of them allow for semi-automatic firing, and they shoot faster and further than spring based guns. They use cartridges with C02, freon, or green gas, which are under pressure, and propel the BB at high speed.
These guns use an electric engine and gears to propel the BB. They have faster firing response times, and most of them allow for full automatic operation where the gun will fire continuously as long as you hold the trigger down. Some of them have incredible rates of fire.