Anti-aircraft guns, also known as Flak Guns/Cannons, are weapons that have the main purpose of, you guessed it, taking down enemy aircraft. Anti-aircraft guns became popular once airplanes became widely used in combat situations, and infantry firearms became ineffective against these airplanes. The type of ammo generally used by farther ranged anti-aircraft guns are shells (generally 40mm-100mm in width), which once airborne explode and send metal shrapnel into the body of an aircraft. This is done in order to increase the chance a fast-moving aircraft gets hit. Shorter ranged anti-aircraft emplacements have higher rates of fire and are usually machine guns with shells ranging in size from 20mm to 40mm. Shorter ranged AA guns have been used heavily in war, and are often mounted on boats, trucks or armored vehicles, and many vehicles have been made specifically for the purpose of taking down aircraft. Larger AA emplacements, are also at least somewhat mobile, usually having wheels.
In addition to the aforementioned type of AA guns, surface-to-air missile systems are also considered to be Anti-aircraft "guns". The missile systems are either mobile or fixed. Mobile missile systems are either tracked or wheeled, and on a truck, tank, or armored vehicle. Some missiles are able to be carried, and shoulder fired, such as the FIM-92 Stinger.