The designation .22 refers to the caliber of the firearm designed to fire the ammunition. Caliber is the interior diameter of the barrel of a gun, in inches. So, a gun with an interior barrel diameter of 22 hundredths of an inch would fire ammunition labeled as .22 caliber.
Compared to other ammunition, .22 caliber is a smaller diameter than most. As such, it is best for hunting small game. This includes coyotes, foxes, groundhogs and other like-sized animals. Firearms of .22 caliber are good to learn to shoot on, due to their small size, low recoil and the relative cheapness and availability of .22 caliber ammunition, according to Guns & Ammo magazine.