A tracer gun, also called a disc gun, is a plastic toy gun which shoots plastic discs roughly the size of a penny.
These discs are often called: "Jet Discs". The firing mechanism is a spring and the magazine can hold up to 20 of these "discs".
Range is about 10 feet, and even then, accuracy is far from dependable. The trigger requires some force. The firing sound is loud, low, and distinctive. Often times the trigger will become loose and will take multiple attempts to shoot.
What's it worth? Take a look at this Tracer Gun price guide: sold listings for a value indication.