Double action means when you pull the trigger it cocks the hammer or striker and then releases it, single action means the slide has to cock the striker or you manually have to cock the hammer and all the trigger does is release the hammer or striker. On a double action only gun you do not have an option to manually cock it, everytime you pull the trigger it will cock and release, on a single action once you cock it and pull the trigger it will release the hammer/striker and in order to do it again you have to cycle the slide or manually cock the hammer. To make things a bit more confusing for ya, the 24/7 and new Mil Pro's are single action guns but if for some reason the round does not go off when you pull the trigger it will revert to a double action pull which means you can simply pull the trigger again and it will cock and fire with that one pull of the trigger.
did ya catch all that ;D