As the name implies, a ‘handgun’ is a small pistol or revolver, designed to be shot with one hand.
A pistol is any ‘automatic’, or ‘semi-automatic’ handgun, that is fed with a ‘magazine’ of bullets. In a semi-automatic pistol (such as a Glock) every time the gun is initially charged and the trigger is depressed, the gun fires one bullet, and then it reloads itself, automatically. In fully automatic pistols (like an Uzi) the gun can fire MULTIPLE times, with one pull off the trigger, and will empty the magazine, if the shooter does not let go of the trigger before the weapon runs empty.
A revolver, or ‘wheel gun’, has a cylinder, which rotates to align with the barrel, BEFORE the gun is fired.