The are different types of handguns. Revolvers have a cylinder that holds the bullets. When the trigger is pulled, it rotates the cylinder to shoot off a round. There are also pistols, which are clip-fed guns that can be semi- or fully-automatic. After a bullet is chambered by pulling the slide back and letting it go forward loading a round in the chamber and the round is fired, the recoil of the bullet has enough energy leaving the muzzle (end of the barrel) to send the slide back to load another bullet.
Semi-automatic is just one pull of the trigger and one shot is fired. Fully-automatic is when the trigger is pulled and multiple rounds are fired while holding down the trigger until the clip is empty.
There are also muzzle-loading handguns which are older and less innovative than revolvers and pistols. Muzzle-loading guns are guns that have one shot; then after that shot is fired, you must load gunpowder, the bullet itself, then a cap on the primary, pull the hammer back, aim and shoot.
