SA, DA - what and how

Simply put Rav,

Having to manually cock the weapon in order to fire it and the weapon will not subsequently cycle itself in order to fire it a second time (Having to pull the hammer back to cock, and then fire, such as certain six shooter revolvers) = Single Action Only or SAO

Pull the trigger, weapon fires and re-cocks automatically with no ability to be able to cock the weapon (Such as a preset striker with no hammer present in order to manually cock the weapon as found in the Glock) = Double action only or DAO

1911, as an example- there are others and having the choice to do either depending on the user = Double Action / Single Action or DA/SA
Generally, with DAO weapons, you can't get a round into the chamber without it cocking internally. The only way to decock would be to fire or dry fire.