A rifle is a firearm that usually requires holding with two hands in order to fire accurately. They generally have a longer range than pistols, and are used in target shooting as well as hunting.
Pistols are smaller than rifles and have a shorter range with less accuracy. In competitions they are held with one hand and can be used to hit targets of various distances in slow and rapid fire precision shooting.
Shotguns are single shoot and reload, or pump action, guns which fire projectiles often full of smaller sub-projectiles. Shotguns are often used in quick reaction competitions, shooting clay targets thrown into the air by a machine.