A revolver is a gun that can be held in one hand and has a revolving cylinder. Most revolvers can fire six bullets without being reloaded.
You can also call a revolver a pistol or a handgun or even a six-shooter. This last nickname comes from the fact that many revolvers have six chambers for bullets in their revolving cylinders. Originally, they were called revolving guns, but the man who invented them, Samuel Colt, preferred the briefer revolver, and it caught on.