When you read articles or hear gun experts talking about revolvers, you'll hear a lot of confusing terminology being thrown around. The facts are actually rather simple. Here are the three basic types of revolver trigger systems.
Single-action-only (SA) This type is the oldest of the systems and requires that the hammer be manually cocked by the shooter for each shot. While slow, it offers a light trigger pull that is conducive to accuracy.
Single-action revolvers such as the Colt Single Action Army were popular in the American West and remain so today among cowboy action competitors and traditionalists.
Double-action/single-action (DA/SA) This system is so named because pulling the trigger will cock and release the hammer. Most double-action revolvers can also be fired in the single-action mode as well. When fired in the double-action mode, most revolvers offer a long, heavy trigger pull that is not conducive to accuracy. For this reason, most double-action shooting is done at close ranges in self-defense scenarios.
Most modern revolvers are of double-action/single-action design, such as the Colt Python, Ruger SP100 and S&W Model 29. This includes even large-bore models that will seldom, if ever, be fired in the double-action mode.
Double-action-only (DAO) In this system, pulling the trigger cocks and releases the hammer. There is no provision for single-action operation. This system is found commonly on small revolvers intended mainly for concealed carry. Most examples have the hammer spur removed for added concealability, while others employ an integral hammer.