If you are referring to autos and not revolvers-------------. The SA auto has a consistent trigger pull from shot to shot. The hammer must be manually cocked for the first shot, by thumb cocking, or by racking the slide. The traditional DA auto cocks the hammer with the first trigger stroke, and there after the hammer is cocked by the slide, all subsequent shots are SA. Some find the transition from DA to SA pull troublesome, some don't. A True DAO has the same trigger cocking action with each pull of the trigger. Then you have the Glocks and XDs that are somewhere in between. The striker is partially cocked by the action of the slide. The trigger pull is consistent from shot to shot with this system as well. Not a good as a SA, but better than the traditional DA. You can learn to live with any or all of the above. Just a matter of preference. I use them all in different applications.
Savage