In simple terms, a potato gun is a device which will launch traditionally a potato. Often homemade, they are capable of firing the projectile in certain cases hundreds of yards.  The tube which the potato shoots out of is usually made from PVC. A firing mechanism will hurl the potato at tremendous speeds down the tube and up into the air. The firing mechanism will generally fall into one of three categories.

Pneumatic Guns

Pneumatic guns basically utilize compressed air to fire the potato. Due the high pressure the gas is stored at, when an opening appears the gas shoots out of its chamber and fires the potato high into the air. Pneumatic guns are less noisy than other guns and are legal in a lot more places than the others. However, due to the high pressure of the gas they can be difficult to build, and in some cases fatal if built incorrectly.
Pneumatic Guns

Combustion Guns

Combustion Guns are much the same as pneumatic guns, with the difference that instead of the gas forcing out of the chamber and providing then thrust, the gas is ignited with a spark causing a mini explosion thus firing the potato out into the sky.  Because of the method of the firing mechanism, a lot less gas is needed, which can make the gun a lot smaller. Because of the explosive nature of the product the legality of this varies from place to place. Read more »