Revolvers got their name from the rotating (or Revolving) cylinder, which contains cartridges. Usually the cylinder holds from 5 to 7 loads, although some .22 caliber revolvers may contain up to 8-10 cartridges. Loads in the cylinder may be reloaded in 2 ways (depending on revolver design) - one by one, as, for example, the Colt PeaceKeeper does (and almost all old-timers), or all simulateounosly - when the cylinder is switched to the side or when the is frame "broke open."
Single Action means, that the Revolver must be manually cocked (and, thus, the cylinder is rotated to the next cartridge) for each shot. This mode was the only one available in all old-time revolvers (such as the Peacekeeper), and is still available in most double-action revolvers. This mode improves accuracy but slows the fire rate.
Double Action for the Revolver means that the hammer for each (including the first) shot is cocked by trigger pull (this action also rotates the cylinder to the next position). This mode speeds up the firing rate and simplifies shooting actions, but greatly increases trigger pull (from 2.2-4.4 lbs usually found in single-actions, to 8.8-12.2 lbs in double-actions).