I've had Airsoft for awhile now. The short description is:
They are 1:1 scale replicas of firearms that shoot a 6mm hard plastic pellet. there are 3 type of guns:
Spring Cocking: single shot, rack the slide every time you want to fire, generally made of plastic magazine loaded. My P228 carries 15 and my Glock 17 carries 20. These are generally reserved for pistols, though there are some shotguns too.
Gas and Gas Blow-Back: These use green gas, and are generally made of metal. Feel and operation is very realistic. Again, usually reserved for pistols, but there are rifles that use the gas, but the go through the gas quickly.
Electric: The SMGs and auto rifles fall in this category. Use a battery pack cleverly disguised within the arm to power the motor. You can even get a replica 20mm chain gun in electric
The spring pistols are anywhere from 25-75 bucks. The gas and gas blowback are around 90-250. The electrics are between 29 and 900. Yep, that $900.00. The (accurate) range on these, depending on projection methods is between 20 feet, and 100 feet. Getting hit with one is akin to a rubber band snapping on your skin, and they will raise a small welt for a short time. Eye danger is there though.
The best site for learning more about them is:
http://www.best.com/~vxl/airsoft
That's where I got started, and they have a large list of retailers there.