They are stunningly realistic looking, so much so I don't go out in the street with them.
Most are made of plastic, the use either 1) spring 2) electricity 3) gas. They are something like paintball, in the sense you can shoot each other with them. They shoot little tiny plastic BBs. Depends, anywhere from probably 50 to 300 feet depending on what type you have
http://www.mfiap.com/airsoft/airsoft.htm <-- the skinny