Pistols 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 the pistol will be ready for the next shot. Cartridges are usually fed from a box magazine, located in the pistol`s handle. Box magazines may contain up to 15 cartridges (or more) in single or double columns, depending on the pistol model, and are easy (and very quick) to reload
Single Action means that the pistol must be manually cocked for the first shot (usually, this is done by pulling the slide - this action cocks the hammer and feeds a cartridge into the chamber). For the second, and all consecutive shots, cocking is done automatically, when recoil force pulls back the slide.
Double action means the hammer is usually cocked by trigger pull for the first shot only; the second and the rest are done in single-action mode. However, first load must be fed in the chamber by the slide pull. Some (most of them - compact) pistols and revolvers employ Double-action-only mode, which cocks the trigger for each shot, thus excluding single-action