Airsoft is a team-based sport, much like paintball, which pits players against each other in non-lethal deathmatch using plastic-bullet or paint-bullet weapons.
In airsoft, these weapons come in a variety of classes - from small pistols to massive machine guns - and are often very realistic - sometimes to the point of being banned in certain countries. The ammunition used is plastic 6mm BB bullets, fired with a velocity of of several hundred feet per second, at realistic firing rates. The bullets are completely harmless, unless you're unlucky enough to get one in the eye - for this reason, most players wear safety goggles. Also, a bullet from a larger gun such as an assault rifle can hurt quite a bit, so most players also wear protective clothes and camoflague.
Tokyo Marui, a japanese company, is the world leader in airsoft weaponry manufacturing. Their lineup includes pistols, shotguns, assault rifles, sniper rifles as well as heavy machineguns: Terrorist faves like the AK47 are available together with US weapons like the Colt M4A1 and european ones like the Steyr AUG.
Airsoft weapons are powered by spring, electricity, or gas. Spring weapons are often very cheap, since they require manual reloading between each shot - you can usually get a spring pistol for something like $20. Gas guns can also be found at low prices but range in price up to about $250 - but the fact that no reloading is required between shots is worth a lot, and many gas guns can even fire full-auto. Spring guns are mostly pistols and shotguns, while all types of guns can be gas-powered because of the high flexibility of the power source. However, gas guns are generally less durable than electric ones.
Most good airsoft guns are powered by electricity - for example, almost every sub-machine gun, assault rifle and sniper rifle you can get are electric. However, there are also electric pistols available. Electric airsoft weapons are called AEGs - Automatic Electric Guns. These weapons are reliable and effective due to their high firing rate and long battery life, however they can be vastly more expensive than their gas or springcounterparts.