A shotgun shell is the standard type of ammunition for a shotgun. It is a cylinder which normally holds either lead shot or a shotgun slug made to be fired from the shotgun. Shotgun shells are usually mostly plastic but have thin brass at the base, though they can be made of solid brass, paper, or other combinations of these materials.
A shotgun shell's size is usually measured in gauge (also known as bore). The most common size of shotgun shell is the 12-gauge. When measuring gauge, a smaller number means a larger shell: a 12-gauge shell is larger than a 20-gauge shell.
Besides lead shot and slugs which are designed to be lethal, there are special less lethal types of shotgun shells which shoot things made to hurt but not kill people. Less lethal shells are most often used by police. These can be loaded with rubber shot or slugs, beanbags, and other things.