A single action trigger on a semiauto, as we know it, ideally is a clean, crisp, pull. There will be little to no take up and no overtravel.
A double action trigger as found on a semiauto or revolver is a long heavy pull that results in a hammer retracting and falling.
On a striker fired pistol, like a Glock, there is a long take up and a definate engagement point that, when passed, results in the striker being released and the pistol firing.
A rolling trigger is a trigger found on a semiautomatic that has a long pull, as on a DA or striker fired gun, but there is no decernable resistance or 'event' which lets the shooter know the gun is about to fire. There is a rolling trigger group that can be fitted to a Walther OSP for International Rapid Fire competition. They are used when engaging multiple targets so the shooter can get a 'rhythm' going when moving from target to target. Rather than aim, click, move, aim, click, move, aim, click... there is a long pull that allows the shooter to move more smoothly from target to target. It is the reason that Bianchi Cup is dominated by revolver shooters, the long pull allows a rhythm to be established. On the Glock 17 and 24 that Dwight Shepard shoots you would see the long trigger pull that Glocks are known for, but the resistance from the beginning to the end of travel does not vary. That is about 1.9 pounds and through that travel the striker is pulled and released. You can't tell that you are pulling back the striker and there is no resistance when it is released.
I assume that you are referring to the do it yourself job on the Glock pistol. The only person that I know that is capable of doing this is Vern Slunaker at S&S Guns in Slidell, La. Dwight is the only person that I know of that has had this work done and it is on race guns. I would not say that Vern is somehow the only person in the world capable of doing that work, you might be another, but he is the only one that I know of. On the Walther OSP it is a factory trigger pack. I do not know if other makers like Hammerli or Pardini offer a rolling trigger option.