Handgun

Please see the wikipedia article on handguns, for general information pertaining to this subject. This article does not seek to explain what a handgun is, but rather how handguns are chosen and what they are generally used for.

Terminology
I'll be using certain largely self-defined terminology in this article.

Pistol Size
Definitions of pistol size vary greatly from language to language, and from producer to producer, I, however, have selected a specific number of sizes to refer to in this article, defined here.

Mouse Gun
A Mousegun can wholly lie in an adult's palm and be invisible if watched from underneath, safe for perhaps a protruding bit or piece that might stick out due to design. The gun is designed not to feature a full-grip and tends to be chambered in low calibers.
 * Example: Remington M95, NAA22S,

Pocket Gun
A pocket can, with an extended magazine or even naturally, support a full grip, provided the shooter doesn't have large hands, and is usually chambered in a short caliber. The gun isn't nearly as small as a Mouse Gun, but only slightly bigger - though just big enough to be fired as a proper pistol.
 * Example: S&W M&P Bodyguard, Ruger LCP, Raven Arms MP-25,

Subcompact
Intended to feel like a normal pistol when used, these pistols often feature a full-grip and if handled exclusively for some time, may even begin to feel like they're normal-size. For shooters with small hands, they may as well be.
 * Example: Walther PPK, S&W M&P Shield, Ruger LCR, S&W J-Frame

Compact
A compact pistol is usually just a slightly trimmed and shortened version of a full-size pistol.
 * Example: Glock 19, S&W M&PC

Full-size
A full-sized pistol is standard service pistol in size and shape.
 * Example: Beretta 92FS, Glock 17, S&W M&P, M1911 (any brand),

Over-sized
An over-sized pistol is a pistol that exists primarily for the sake of novelty or for maximum precision, such as with hunting revolvers.
 * Example: Desert Eagle.

Caliber
Caliber, in the world of firearms, is a confusing term that can mean any number of different things. We're not going to be touching upon that further in this article, but instead we're just going to quickly make a division of calibers, largely referring to the specific families of cartridges that exist in these groups.

Small Caliber
Small Calibers are calibers that are, by modern standards, not considered good combat calibers. Many of these have historically been used as combat calibers, but most of them are now, either outdated-, novelty- or varmint calibers.
 * Includes: .22 LR, .25 ACP, .32 ACP and many similar

Short Calibers
I'm certain that there are other calibers that belong in this group, but there's only really one that presently comes to mind. This group contains combat calibers that have been reduced in size and power, but retain lethality, with less penetrative power, making them ideal self-defense guns, especially for concealable handguns.
 * Includes: .380 ACP

Medium Caliber
These are modern combat calibers, wholly considered to be adequate for combat and killing by modern today's standards, and are not commonly used for hunting. They tend to be relatively light on the recoil, but provide very respectable range and retain very adequate penetrative power. Large Caliber
 * Includes: 9mm (both common variants), .40 S&W, .38 Special,

Large Calibers have their pros and cons, but when it comes down to it, they tend to be reliable and very well functioning combat rounds, but they tend to be so heavy (and yet still short) that they have limited reach, but not powerful enough that the recoil leaves the shooter reeling.
 * Includes: 10mm Auto, .45 ACP, .45 GAP,

Magnums
Magnum rounds tend to be far too powerful to effectively use in a firefight, but they are excellent for pistol hunting, and for bear-defense, though a trained shooter can achieve reliability with these rounds and they do have some self-defense application - after all, one of the below rounds was used as a service round for the police in large portions of the USA.
 * Includes: .44 Magnum, .357 Magnum and various other magnum loads

Over-Sized Calibers
These Calibers are too large to be effective in a pistol-sized platform, and short of novelty use or for self-defense against bears, they have no real purpose.
 * Includes: .50 AE, .500 S&W Magnum

Types
There is a number of primary categories of handguns.

Sidearm
The word "sidearm" describes a weapon that is carried in the line of duty (it doesn't actually have to be a firearm, but in modern day, it traditionally is), as a deterrent against assault and a self-defense weapon, but never to be confused with a primary weapon. While the user of a sidearm has to be willing to engage in combat with the sidearm if necessary, it should never be the first choice of weapon, if there is time to plan.

Scenarios and Examples
I'll provide a few scenarios below, showcasing typical situations where a given individual might need a sidearm, and what qualities the sidearm should have.
 * Low risk - The sidearm should be a full-sized revolver, chambered in medium, large or magnum caliber, with a fully accessible external hammer and night-sights, ideally used by security guards or police officers who have some sort of desk-duty or mainly need the gun for symbolism. There are impossibly many examples...
 * Medium risk - the sidearm should be an auto-loader with an external hammer, be full-sized with a single-stack magazine, night-sights and chambered in either medium or large caliber with an external hammer, preferably as many safeties as the weapon can muster, for use by patrolling security and police, who could easily get into a situation where the weapon is stolen and possibly risk having it used against them. A Colt 1911 or a Sig Sauer P210 are good examples.
 * High risk - the sidearm should be a full-sized striker-fired firearm with a double-stack magazine, chambered in medium or large caliber, with night-sights, used by some security forces and police officers in particularly high-risk areas, when it is of utmost importance to be able to draw and shoot in a split-second. The Glock 17 is the best known example.
 * Low-Profile - The sidearm should be a compact, or even sub-compact revolver or auto-loader, with an external hammer, chambered in medium, large or magnum. Used by police detectives primarily, but also by body-guards trying to keep a low-profile and having their gun show at every turn. J-frames and Walther PPK are excellent examples.
 * War-Zone - The sidearm should be a compact (if possible) autoloader with an external hammer, and feature a double-stack magazine, have night-sights and be chambered in medium or large caliber. Sidearms are traditionally carried by all officers of all types in most military forces. They carry these both in and out of garrison, so that they can exert authority over their lessors who typically are only armed when on duty, but also to prevent and deter assassination attempts. The Para-Ordnance 1911, H&K USP and Sig Sauer P220-series are good examples.

Secondary Weapon
Using a pistol as a secondary weapon was popularized in computer games, but people often ended up questioning this, as pistols were often portrayed as "weak" and largely ineffective weapons in simulations, leading to a general misunderstanding that Pistols are simply not effective compared to other weapons - this is naturally a misconception, as pistols have a very significant application as potential secondary weapons, specifically to be used in close-quarters, where other weapons may require too much space (or two many hands) to handle, or would over-penetrate due to short range (thus not inflicting adequate damage to the target).

Scenarios and Examples
I'll provide a few scenarios below, showcasing typical situations where a given individual might need a pistol for a secondary weapon, and what qualities the secondary should have.
 * Combat Arms - The backup should be a compact striker-fired autoloader with night-sights, a double-stack magazine and chambered in medium caliber. The Secondary Pistol offers the soldier an effective weapon to fight in close-quarters with more agility and more lethality than could be obtained with a typical military firearm. To a soldier on the front-line, a pistol can be the difference between life or death - it can help a separated soldier avoid capture, allow a soldier with a jammed weapon to fire back, offer some personal security if the soldier loses their primary weapon, see a soldier to safety when he has expended his ammunition for his primary weapon and it is possible to draw in confined quarters, such as inside of a wrecked vehicle. The Glock 19 is the best known example.
 * SWAT - Pretty much the same as above...

Holdout
A Holdout is a type of weapon used as a last-line of defense, typically by civilians. Some military scouts have been known to use holdouts as well.

Scenarios and Examples
I'll provide a few scenarios below, showcasing typical situations where a given individual might need a Holdout, and what qualities the Holdout should have.
 * Civilian (Low-Threat) - A sub-compact revolver with a snubbed external hammer, in medium, large or magnum caliber. For the untrained, a revolver with a snubbed hammer offers all options, but few things that can go wrong. The J-Frames and Ruger LCR are excellent examples.
 * Civilian (high-threat) A single-stack sub-compact or pocket-sized autoloader with an external hammer in short, medium or large caliber. For the individual who plans on carrying a spare magazine, and expects the possibility of getting into a lengthy firefight. It is important to commit to this choice by actually carrying at least one spare magazine, as magazines introduce the possibility of magazine failure, which further complicates matters. Walther PPK is an excellent example.
 * Off-duty - A single-stack sub-compact or Pocket-sized striker-fired autoloader in short, medium or large caliber. For those who are trained in the use of firearms, and ordinarily use a striker-fired firearm. Train as you fight, fight as you train. S&W Bodyguard or Shield are good examples.
 * Minimalist - A Mouse-Sized gun of any kind in as high caliber as can be managed. Those who wear outfits that cannot be used to conceal a larger weapon, or women with purses can often benefit from the use of a Mouse-Sized gun, that can fit in a back-pocket, or in a purse with no issue. It isn't much, but it is certainly better than death...