Handguns are mostly small, lightweight,  and provide firepower  for defensive / offensive situations and hunting.  Handguns are most commonly divided into semi-automatics (a/k/a pistols) and revolvers.

Semi-automatics use part of the energy produced by burning cartridge powder to remove the used cartridge from the chamber, cock the hammer (or striker) and load a new cartridge in the chamber, so that the pistol will be ready for the next shot. Cartridges are usually fed from a magazine located in the pistol's handle.  Magazines could  hold 6, 10, 15, or more cartridges in a single or double column, depending on the model of the pistol.

Revolvers get that name from the rotating (i.e., revolving) cylinder, which typically contains 5 to 7 cartridges, however, certain models could be designed to hold more less.  Cartridges may be loaded into the revolver's cylinder 2 ways, again, depending on  design.  They may only hold one cartridge at a time by one, or several at one time.

Single action and Double action styles are possible in both revolvers and semi-automatics.

Single action revolvers must be manually cocked for each shot and is a common alternative feature in double-action revolvers. This mode improves accuracy but slows the overall time to actually fire the gun.

Single action semi-automatic pistols must be manually cocked for the first shot.  Typically,  this is accomplished when the slide is pulled back, resulting in a cocked hammer, while also feeding a cartridge into the chamber. For the second, and all consecutive shots, cocking is done automatically, when recoil force pulls back the slide, ejecting the spent cartridge and loading a fresh one from the magazine into the chamber..

Double action revolvers means that the hammer for each shot is cocked by pulling the  trigger.  This action simultaneously rotates the cylinder to the next position. This mode speeds up the firing rate and simplifies shooting actions.  The double action feature does require increased trigger pull from single actions revolvers.

Double action semi-automatic pistols usually cock the hammer with the  trigger pull for the first shot only after that first load has been fed in the chamber by the slide pull.  The second and remaining cartridges are fired in single-action mode.  Some semi-automatics and revolvers have a double-action-only (DOA) mode, which requires the trigger pull to cock the gun for each shot.