Some revolvers are single action and some are double action, what exactly is the difference between the two types?
TDLR : Single actions only fire the revolver, double actions both cock the hammer and fire.
The way that revolvers work is that each time it is fired, the cylinder must be rotated and the hammer cocked so that it can be dropped onto the primer to fire the bullet.
To do this with a single action revolver the user has to manually cock the hammer back before pulling the trigger while pulling the trigger on a double action revolver cocks the hammer and then fires the revolver by dropping the hammer onto the primer.
The downside of not having to manually cock the hammer with a double action is that the the trigger pull is much heavier and that leads one to be less accurate.
Most double action revolvers however do have the ability to manually cock the hammer for extra accuracy.
Double action revolvers that lack the ability to manually cock the hammer are called double action only.
In addition there are a few double action revolvers such as the Webley–Fosbery Automatic Revolver where the recoil is used to rotate the cylinder and cock the hammer to avoid the heavy trigger pull.
Striker-fired pistols too, can be either "double action" or "single action." Usually the first shot in a magazine will require you to manually chamber the round, after which a pistol is then "single action." In cases where you don't manually chamber the round, the trigger pull is a longer, heavier, double action pull.