For starters, there are two different "actions" a revolver trigger can possibly perform:
1) Releasing the hammer for firing once it's cocked;
2) Cocking the hammer while at the same time spinning the cylinder to the next round.
Early revolvers could only do the first action; they had to be cocked by hand (or thumb, or whatever) before each trigger pull and fire.
These were thus "single action".
Double action revolvers can perform both, so you can fire until the gun runs dry by just pulling the trigger repeatedly - sort of like a semi-auto, but with a difference:
The DA trigger pull is always a "long stroke" and somewhat heavy, because you're hauling the hammer back against mainspring pressure. DA triggers run anywhere from about 9lbs to 14 or so.
SA triggers can be as light as a pound or two, but under 3 is considered unsafe in a "street defense gun". And the SA trigger movement is very short; on a good gun, it'll feel "like a glass rod breaking" with no creep, roughness, etc.
Most DA guns have a hammer that can be manually cocked to allow an SA shot when desired. In other words, for a defense gun, you can whip it out and take a quick DA shot at close range, or if the problem is at longer range, you can cock it for greater accuracy.
Another issue is that almost all SA guns made today are made to look like and (mostly) work like an "old west cowboy gun" such as the 1873 Colt SAA "Peacemaker". Hence, in addition to having to cock them for every shot, reloading is comparatively slow. The cylinder can't be "swung out" like a modern DA gun, which on modern DAs lets you empty all six (or whatever) cylinders at once and gives you access to all of them for reloading.
On classic SA guns, bullets are loaded one at a time into the cylinder via a "loading gate" - the cylinder cannot "swing free".
The Ruger SP101, GP100, Redhawk and Super Redhawk series are all DA guns. All have hammers that can be cocked for SA firing except a few variants of the SP101.
Ruger's Blackhawk, Single Six, Bisley Blackhawk and Super Blackhawk are SA guns with fairly modern sights. The Vaquero, Bisley Vaquero and Bearcat have more primitive "cowboy type sights" making them look even more like genuine old-west gear.
SA guns actually have some advantages. When done up with modern metallurgy, they can be much stronger than most DA guns because the cylinder is locked in place. The grip design was originally made for comfortable one-handed shooting (from horseback
). And some people just like how they feel and shoot; recoil control happens from the grip "rolling up in your hand". The Bisley-type grip in particular has an excellent rep for controlling big recoil way out past .44Mag power levels.
Still and all, most people use SAs only for hunting, wilderness defense, target and sports like Cowboy Action Shooting. DAs are the modern defensive revolvers, although there'll always be a few die-hards packing SAs because they've gotten very good at 'em
.
What else...all of the Ruger SAs made since 1973 have a modern safety system built in that prevents firing unless your finger is pulling the trigger. Called a "transfer bar safety", it's similar to safeties found on DA revolvers, so like almost all DA guns, it's safe to carry them with a live round in every chamber. Genuine 19th-century cowboy guns lacked such an automatic safety, so they had to be carried with one less round and the hammer down on the empty cylinder. Tradition had them rolling a $20 bill into the unused cylinder, which meant that the gun was safe to carry when the hammer was "on the money" - which is where we get that term. The other term we still have from old non-safety SA wheelguns is "going off half cocked" which should give you some clue as to how common accidents were if we're still using the terms today without even knowing they're old revolver terms.
Anyways, what it means is, internally the SA Rugers are not "totally authentic". Which as far as I'm concerned, is a good thing
.