There are two common types of revolvers available today. The single-action revolver requires shooters to cock the hammer by hand before each shot. The trigger on a single-action revolver will not function until the gun is cocked.
The double-action revolver can fire in one of two ways. The shooter squeezes the trigger to cock and release the hammer. This cocks and fires the gun. The shooter also can first cock the hammer and then squeeze the trigger, just like with a single-action revolver.
Revolvers cannot be fired unless the trigger is squeezed. Generally speaking, old revolvers do not have safeties. Therefore, you must use extra care when handling older revolvers because they can fire if they are dropped when loaded!