A single-action revolver is a small handheld gun in which the hammer must be manually cocked before pulling the trigger. This action causes the cylinder containing the ammunition to turn, lining up a new round with the barrel.
The other common type of revolver is called a double-action revolver. This type of revolver only requires a single trigger pull without manually cocking the hammer. The trigger pull cocks the hammer, spins the cylinder to the next position and fires the weapon. The trigger pull generally requires a little more force on a double-action revolver but can be fired more quickly.