A revolver works by having several firing chambers arranged in a circle in a cylindrical block that are brought into alignment with the firing mechanism and barrel one at a time. In contrast, other repeating firearms, such as lever-action, pump-action, and semi-automatic, have a single firing chamber and a mechanism to load and extract cartridges into it.
A single-action revolver requires the hammer to be pulled back by hand before each shot, which also revolves the cylinder. This leaves the trigger with just one "single action" left to perform - releasing the hammer to fire the shot - so the force and distance required to pull the trigger can be minimal.
In contrast, with a self-cocking revolver, one long squeeze of the trigger pulls back the hammer and revolves the cylinder, then finally fires the shot. They can generally be fired faster than a single-action, but with reduced accuracy in the hands of most shooters.
Most revolvers do not come with accessory rails, which are used for mounting lights and lasers, except for the Smith & Wesson M&P R8 (.357 Magnum), Smith & Wesson Model 325 Thunder Ranch (.45 ACP), and all versions of the Chiappa Rhino (.357 Magnum, 9mm Parabellum, .40 S&W, or 9x21mm) except for the 2" model, respectively. However, certain revolvers, such as the Taurus Judge and Charter Arms revolvers, can be fitted with accessory rails.
Most commonly, such guns have a 5- or 6-shot capacity, hence the common names of "six-gun" or "six-shooter". However, some revolvers have a 7- to 10-shot capacity, often depending on the caliber, and at least one revolver has a 12-shot capacity (the US Fire Arms Model 12/22). Each chamber has to be reloaded manually, which makes reloading a revolver a much slower procedure than reloading a semi-automatic pistol.
The alternatives are a replaceable cylinder, a speedloader which can reload all chambers at once, or a moon clip that holds a full load (or even half of one in the case of a half-moon clip) of ammunition and that is inserted along with the ammunition. In revolvers chambered for rimless cartridges, moon clips are normally required, though a few revolvers have been built with a special extractor with individual tabs to engage rimless cartridges.
Another product known as a "speedstrip" cannot reload a completely empty revolver as rapidly as a speedloader, but is less expensive, flatter, and more flexible when it comes to partial reloads.
Compared to autoloading handguns, a revolver is often much simpler to operate and may have greater reliability. For example, should a semiautomatic pistol fail to fire, clearing the chamber requires manually cycling the action to remove the errant round, as cycling the action normally depends on the energy of a cartridge firing.
With a revolver, this is not necessary as none of the energy for cycling the revolver comes from the firing of the cartridge, but is supplied by the user either through cocking the hammer or, in a double action design, by just squeezing the trigger. Another significant advantage of revolvers is superior ergonomics, particularly for users with small hands. A revolver's grip doesn't hold a magazine, and it can be designed or customized much more than the grip of a typical semi-automatic.
Partially because of these reasons, revolvers still hold significant market share as concealed carry and home-defense weapons.(wikipedia.org)