We have a video below showing and demonstrating it in action so I will be brief with the write-up.
Double Action is found on most automatics and some revolvers. DA means when you pull the trigger, aside from a manual safety, there is direct contact to the hammer and the weapon is fire-able. You pull trigger, gun goes bang. DA almost always has a long and heavy pull, especially so in the case of revolvers. One more reason why you should ALWAYS check your chamber. If there is one in the pipe on your DA pistol and you pull the trigger, it will go off.
Single action is found on most 1911’s, “Cowboy Action” revolvers and generally any weapon that needs to be cocked first before firing. In the case of many semi-autos, they become SA after you send one downrange. Single action means the trigger will do absolutely nothing for you UNLESS a round is chambered and the hammer (or striker on hammerless pistols) is cocked. 1911 for example (most not all), if the hammer is forward the trigger goes nowhere, however when you rack a round in the chamber and the hammer is back, it is a VERY short and drastically lighter pull on the trigger to make the gun go boom. This also aids in better accuracy since there is less force required to fire giving less room for flinching and jerking.
So when shopping, analyzing, looking…whatever you are doing. If you want a certain pistol for a certain job this can aid you in choosing the right “tool for the job”. Your proficiency, use and needs should dictate which one is right for whatever you are looking to do, hopefully this helps.