Single actions, as typically seen riding in John Wayne's holster in the westerns, normally load from a gate on the side of the cylinder and will only fire if the hammer is fully cocked. Double actions load by either swinging the cylinder out of the frame or else the frame hinges in front of the cylinder and the front end tips up, allowing access to the cylinder. D/A's also will fire with a long trigger pull without the hammer needing to be manually cocked. Hope that helps, and good shooting. E~WV

On many double action revolvers, the hammer may also be cocked manually so that the trigger may be pulled to release the hammer (single action mode).

Don't forget the semi-auto pistols. Single actions (ala 1911s) also require the hammer be cocked before the gun can be fired.

With double action semi-auto pistols, the trigger must be pulled to both set (can't write C0ck on this website) the hammer and release it. This must be done on every shot.

On double-single action semi-autos the hammer may or may not be cocked on the first round. If it is not cocked then the first shot will be double action (set and release the hammer). If the hammer is set, either manually or by racking the slide, the first shot is single action (trigger releases the hammer). Subsequent shots are single action since the slide sets the hammer. Hence double-single.

Accidentally releasing a hammer before it is fully cocked can fire many guns, as a friend found out while holding a .44 magnum pointed at his leg.

Single actions typically have the advantage of a light crisp trigger pull. Double actions have the advantage of firing the gun only by pulling the trigger (even on a misfire on a semi-auto), many with an option of also firing single action.