Single action-
When referring to a revolver it means that you have to pull the hammer back with your hand or thumb every time you want to fire a shot(cock it and then pull the trigger) Examples are Colt Single Action Army(SAA) Ruger Blackhawks, Ruger Vaquero, Ruger Super Blackhawk and MOST common percussion cap revolvers.
When referring to a Semi Automatic pistol it means that you have to manually cock the hammer for the first shot but not on subsequent shots because the slide cocks the hammer for you. Examples are Colt 1911 style pistols and Browning High Powers.
Double Action
For revolvers it means that when you pull the trigger it cocks the hammer and then fires the pistol (albeit very quickly). For a revolver this must be done for every shot. Most double action revolvers can also be fired in single action mode as well. Examples (in my experience) are Most revolvers with a swing out cylinder.
For a Semi Automatic pistol means that when the hammer is not cocked you can pull the trigger and it will cock and fire the pistol. you do not have to cock and fire the pistol with the trigger for every shot because the slide cocks the hammer when it cycles. This type of action is sometimes referred to a Double Action/Single Action (DA/SA) Example FEG P9
There is another option which is Double Action Only (DAO) this is simply a revolver or pistol that can't be fired in single action. Examples Glock pistols and I think that Berretta makes a version of the 92 that is DOA and the Browning BDA is DAO.
When referring to a revolver it means that you have to pull the hammer back with your hand or thumb every time you want to fire a shot(cock it and then pull the trigger) Examples are Colt Single Action Army(SAA) Ruger Blackhawks, Ruger Vaquero, Ruger Super Blackhawk and MOST common percussion cap revolvers.
When referring to a Semi Automatic pistol it means that you have to manually cock the hammer for the first shot but not on subsequent shots because the slide cocks the hammer for you. Examples are Colt 1911 style pistols and Browning High Powers.
Double Action
For revolvers it means that when you pull the trigger it cocks the hammer and then fires the pistol (albeit very quickly). For a revolver this must be done for every shot. Most double action revolvers can also be fired in single action mode as well. Examples (in my experience) are Most revolvers with a swing out cylinder.
For a Semi Automatic pistol means that when the hammer is not cocked you can pull the trigger and it will cock and fire the pistol. you do not have to cock and fire the pistol with the trigger for every shot because the slide cocks the hammer when it cycles. This type of action is sometimes referred to a Double Action/Single Action (DA/SA) Example FEG P9
There is another option which is Double Action Only (DAO) this is simply a revolver or pistol that can't be fired in single action. Examples Glock pistols and I think that Berretta makes a version of the 92 that is DOA and the Browning BDA is DAO.