It get's real confusing when you get into auto-loading pistols instead of revolvers. Took me forever to figure it out, especially when you have DAO thrown into the mix.
On top of what everyone else has already stated, one significant difference between double-action and single-action is the trigger is usually easier to pull on a single-action, because the hammer is already cocked. On a double-action, you are cocking the hammer as you are pulling the trigger, so it's harder to pull. Double-action allows you to fire in single-action mode, by cocking the hammer first, just like a single-action.
For autoloaders: it get's wierd:
All auto-loaders eject the old round and load a new round when the slide goes back.
The slide will go back when a round is fired, or you can pull it back manually, this is called "racking" the slide. One example of a time when you want to rack the slide is to load a round from the magazine if there is not one already loaded.
Single and double-action autoloaders have a hammer that is cocked when the slide goes back.
If the hammer is not cocked on a single-action, it will need to be cocked before the trigger will do anything, either by racking the slide, or by manually cocking it.
A double-action auto-loader will cock the hammer for you the first time you pull the trigger, each shot after that the hammer will be ready because the previous shot sent the slide back, cocking the hammer and loading a new round.
Here is the confusing part:
Double-action-only (DAO) auto-loaders are not cocked by the slide, nor can you cock them manually. Each time you pull the trigger, you are cocking and releasing the hammer. This makes for a harder trigger pull each time. Actually, there isn't really a "hammer" to speak of (or see), it's a different mechanism in this case.
wikipedia can be edited by anyone to that one guy
Yes it can, have you ever tried it? Try posting some nonsense on that page linked above, then hold your breath until someone corrects it. You won't even pass out.