A shotgun shell is a “a self-contained cartridge typically loaded with multiple metallic ‘shot’, which are small, generally spherical projectiles.” Shotguns are also capable of firing a single projectile, called a ‘slug’.
A shotgun shell is cased in plastic with a brass base containing the primer.
Starting at the brass, the layers of a shotgun shell are brass, propellant, over-powder wad, shot wad, shot pellets (or slug), over-shot wad, and top crimping.
The brass base of the shell is thick enough to hold the primer, which is longer than those used for rifle and pistol ammunition.