(Photos by Robert Brown)
Except for certain specialized pieces, modern one-hand firearms are designed to fire several shots between loadings. They are classified according to the means by which they do this.
Contrary to common literary misusage, "revolver" is not a generic term for handguns, but refers to a specific type.
A revolver incorporates a cylinder, which is bored through to create a number of chambers. (Writers sometimes confuse these.) Cartridges are loaded into these chambers. The most common number is six, but there are many variations, especially in smaller calibers.
This cylinder is rotated by the gun's mechanism so that the chambers revolve (duh) around a central axis, thus bringing the cartridges one by one into line with the barrel.
To fire a revolver, the hammer must be brought back against spring pressure, thus activating the mechanism that rotates the cylinder. As the next chamber snaps into position, the hammer can then be dropped to fire the weapon.
The hammer can be pulled back (cocked) by the thumb, and then released by pulling the trigger. This is called single-action firing.
However, modern police and defense revolvers can also be fired double-action. This means the user simply pulls back on the trigger and the mechanism rotates the cylinder, cocks the hammer, and then fires, all in one continuous process. This is faster, but requires a much longer, harder trigger pull, and so it is harder to shoot double-action with any accuracy.
Firing a revolver, then, requires either pulling back the hammer or a long hard trigger pull, neither of which is likely to occur accidentally. For this reason, MODERN REVOLVERS DO NOT HAVE SAFETIES.
(That is, they do not have manually operated safety catches. All modern revolvers incorporate some sort of safety mechanism that blocks the firing pin until the trigger is pulled, so that the gun will not go off if accidentally dropped - for which reason people who carry a modern revolver with the hammer down on an empty chamber are being silly nitwits, though with the old-fashioned cowboy single-action this was indeed an intelligent precaution. The dropped gun that goes off is another very common error in literature and film.)
For reloading, most modern revolvers are the swing-out type, as illustrated; operating a thumb button allows the cylinder to swing out to the side, where an ejector rod is used to kick the empty cartridge cases out. Some older designs break open at the top, while others, such as the classic Colt Frontier sixgun, don't break open at all, but have to be reloaded one at a time through a clumsy loading gate on the side.
One other point: some firearms purists insist that "pistol" should not be used to refer to a revolver. A pistol, they say, is one thing; a revolver is something else. Ballocks to that, and to them. Samuel Colt called his invention a "revolving pistol" and he certainly should have known. If it was good enough for him it's good enough for us.
The other major type of handgun is the semi-automatic pistol, commonly called "automatic" or simply "auto".
Strictly speaking, only "semi-automatic" or "semi-auto" is correct; "automatic" properly refers only to firearms that fire continuously when the trigger is held down - that is, machine guns. However, "automatic" and "auto" are very often used, correctly or not, even by knowledgeable firearms users; and so there would be nothing wrong with a writer having his/her characters use these terms. In fact the longer, more proper term is not very widely used by the sort of people who actually use handguns routinely: cops, criminals, etc. Definitely, in any sort of period piece - anything set in the first half of the twentieth century, on up through the fifties to mid-sixties - the characters should say "automatic" because that's what people said back then; the more correct term didn't really start to catch on until more recently.
Just to make this page a little shorter, we will use "auto" to refer to the semi-automatic pistol. After all, there are no fully automatic one-hand guns - barring a few rare designs that were never successful - because it's simply not possible to control burst fire in a pistol.
Now then. An auto has only one chamber, which is an integral part of the barrel. It has no cylinder, and therefore cannot be mistaken for a revolver.
Instead, the cartridges are loaded into a metal magazine or clip (technically "magazine" is correct, but "clip" is more commonly heard). This clip is then inserted into the butt of the pistol. The clip contains a spring, which pushes the cartridges upward for feeding into the chamber.
When the auto is fired, the force of the recoil is used to drive back the slide. (There are several means of doing this, but we needn't get into such details here.) As the slide flies backward, the empty cartridge case is extracted from the chamber and thrown out through the ejector port. The rearward force of the slide also cocks the hammer.
(Some autos do not have hammers, but use a spring-loaded firing pin, like a rifle. The basic principle is still the same.)
Now the slide is slammed forward by the pressure of a powerful spring. As it moves forward, it picks up the top cartridge in the clip and pushes it into the chamber. Depending on the power of the weapon, it may also be locked into place by some mechanism. Once the slide stops moving, the pistol is ready to fire.
Note that at this point the hammer is fully cocked; a simple pressure on the trigger will cause the pistol to go off. For this reason, most autos of older design DO have safety catches of some sort.
However, in recent years the trend has been toward auto pistols which do not cock themselves in this way, but have to be fired double-action, like a revolver. This has mostly been in response to police departments worried about lawsuits; but because the cops represent such a huge chunk of the market, their preferences have had a major effect on the overall picture. For one very important example, the Glock pistol, widely used by police and favored by some civilians as well - no accounting for tastes I suppose - does not incorporate anything like the traditional sort of manual safety.
(The Glock is basically what you'd get if Microsoft went into the pistol business. But I digress.)
So the best way to stay out of trouble is to remember this: if your character pulls out a "semi-automatic", without further specification, you're safe enough in having him/her flick off the safety - at least if he/she is a private citizen. However, if you're going to get specific and have him/her whip out a Shmeggegi & Ginzorninplad 11mm. Mark XIII, then you better do some homework and find out whether that particular model HAS a flickoffable safety.
And if he/she is any kind of cop, and the setting is contemporary, then he/she probably won't be jiving around with any safeties.
But if your character is packing any sort of revolver whatever, IT HASN'T GOT A SAFETY so don't have him/her flipping it on/off if you don't want to look like an ignoramus.
All the above remarks are very general; exceptions could be pointed out to many of these statements. However, this should give you a basic idea of the terms and what they mean.