TYPES OF GUNS
SPRING

1911 Style Spring PistolSpring powered guns are cocked by hand and fire one shot at at time. They employ few moving parts which make them very durable and dependable. The downside is that they have a slow rate of fire so you have to make every shot count.  Spring powered airsoft guns are generally the least expensive, thus this is what most airsoft players start with.  Many high power airsoft sniper rifles are also spring powered. They are usually operated by a bolt action lever that simulatneously chargest the spring and loads the next BB.  These rifles often shoot as high as 450 FPS or higher using 0.20g BBs.  At those velocities, a heavier BBs is usually recommended to maintain accuracy.

ELECTRIC (AEG)

KWA KM4 Ris Airsoft RifleAEG's (Auto Electric Guns) are battery powered and usually feature both semi and full auto functions. An electric motor turns gears in a gear box which in turn powers the firing mechanism. With a charged battery in full auto mode AEG's will pump out BBs for as long as you hold down the trigger. In semi-auto mode they will fire one shot for each time the trigger is pressed. No cocking required. Most airsoft rifles are AEG's. AEG generally refers to airsoft guns with a full metal gearbox. Low powered electric guns with plastic gears are usually referred to as LPEGS.

GAS

STI Tac Master Airsoft PistolGreen gas and CO2 is most popular for pistols but a there are also many gas and CO2 powered rifles on the market. Green gas is a form of compressed propane gas which is injected into the gun. When the trigger is pulled a burst of gas is released which propels each shot. Most green gas guns are semi auto only but a few come in a full/semi auto version as well. CO2 airsoft guns work the same way, but use cartridges of compressed CO2 instead.  Most higher-end gas pistols have a feature called "blowback."  Blowback airsoft guns have a slide that moves back and forth when the gun is fired, just like a real semi-automatic pistol.  The blowback action adds a small amount of recoil which increases the realism of these airsoft guns.