A firearm is a kinetic energy weapon that fires either a single or multiple projectiles propelled at high velocity by the gases produced by action of the rapid confined burning of a propellant. This process of rapid burning is technically known as deflagration. In older firearms, this propellant was typically gunpowder, but modern firearms use smokeless powder or other propellants.

The term gun is often used as a synonym, but technically only refers to smoothbore firearms, machine guns, and naval artillery. In other terms, a rifle is not a gun.

For handguns, long guns and historical cannon the projectile is a bullet or cannonball, fired by the burning of the propellant, but it does not generally contain explosives itself. For modern artillery the projectile is a shell, containing explosives.

Technically speaking, the projectile is the weapon and the firearm is the weapon platform. In some cases the firearm can be used as a weapon without firing a projectile, although this is generally a secondary method of attack. For example, long guns such as rifles, muskets, and assault rifles can have bayonets fixed to them, becoming in effect a spear or pike. The stock of a long gun can be used as a club. It is also possible to strike someone with the barrel of a handgun or grasp it by the barrel and strike someone with the butt. This is called "pistol whipping".

There are two basic categories of firearms: artillery and small arms. Small arms are generally smaller man-portable firearms, while artillery are larger firearms. Strictly speaking, not all small arms are weapons, but it is the most convenient category under which to group firearms other than artillery.

The smallest of all small arms is the handgun. Handguns with a fixed firing chamber are pistols. A common handgun design that lacks a fixed firing chamber is the revolver, which holds a number of firing chambers in a revolving cylinder.

Pistols are often single-shot, but can also be capable of holding multiple rounds. Most pistols that hold multiple rounds are semi-automatic.

A Derringer is a short barreled handgun, with one or two barrels, that must be manually reloaded after being fired. Duelling pistols exist, as do fully automatic machine pistols. However, the distinction between a machine pistol and a submachine gun is often blurred.

Handguns have a variety of purposes. Handguns are small and concealable, and are thus the first choice for personal protection. In the military, handguns are usually used by those who don't need larger firearms such as general and staff officers, and also by those for whom there is no room for a full-sized rifle, such as armored vehicle and air crews. In this role, they often compete with the carbine. Outside the military world, handguns are the usual armament for police (in those jurisdictions where the police are usually armed) and, where legal, for private citizens while in public. Private citizens who go about in public armed usually carry concealed handguns, since an unconcealed weapon would attract undue attention and would be less secure. In jurisdictions where handguns are not illegal, their concealment usually is, although it is possible to attain a concealed handgun license. This is considered a political issue; see Gun politics for more information.

Handguns are also used for various hunting and sporting purposes.

Most modern long guns are either rifles or shotguns. Historically, a long smoothbore firearm was known as a musket. A rifle has a rifled barrel that fires single bullets while a shotgun fires large packets of shot, a single slug, or a specialty round (tear gas, bolo, lead powder, etc). Rifles are often built for accuracy and long range, and are usually aimed, while shotguns are usually designed to quickly hit a moving target. Rifles have a small impact area but a long range and high accuracy, while shotguns have a larger impact area with less range and accuracy. However, the larger impact area can have the same effect as greater accuracy, since with a shotgun, you do not have to necessarily be lined up with the target in order to hit it.

A short rifle is usually called a carbine (pronounced "car bean").

Rifles and shotguns have many uses, both military and civilian. In the military, rifles are the main armament of modern infantry. Shotguns are also used for some limited applications. Carbines are usually used by those who don't need larger firearms such as field officers, and also by those for whom there is no room for a full-sized rifle, such as armored vehicle and air crews.

In the civilian world, rifles and shotguns are commonly used for hunting and to defend a home or place of business. Usually land game are hunted with rifles (although shotguns can be used, deer hunting with a shotgun usually employs the use of buckshot) while birds are hunted with shotguns. Shotguns are preferred for defending a home or business due to their shorter range and lesser penetration of walls and other structures, making them safer and less damaging.

A machine gun is a large, fully automatic firearm used almost exclusively by the military for large scale functions. They are primarily defensive weapons, mainly because of the difficulty involved in moving and placing them, though light machine guns (such as the US military's M249 Squad Automatic Weapon) often are wielded by a single infantryman in a typical squad; they provides large volumes of fire typically used for suppression and cover.

The cannon is loaded with gunpowder and the cannonball through the muzzle, while a fuse is placed at the rear. This fuse is lighted, causing the gunpowder to ignite and propel the cannonball. Most cannon were land or ship-based guns, although hand cannon also existed. In military use, the standard cannon was tremendously powerful, while hand cannon were tremendously useless.

Muzzle-loading muskets were the first small arms developed. Invariably, the firearm was loaded through the muzzle with both gunpowder and the bullet (usually referred to as a "ball"). Muzzleloaders still exist today and have their respective enthusiasts. However, muzzleloaders have to be reloaded after every shot, and a skilled archer could fire multiple arrows faster than most early muskets could be reloaded and fired. This became less of a concern as technology progressed, and muzzleloaders became the standard armament of modern militaries by the 18th century. Before that point the military effectiveness of muzzleloaders was hindered by the low reloading speed and, in early times, the high risk the weapon still posed to the person firing it.

Matchlocks were the first and simplest firing mechanisms developed. With a matchlock, the powder is ignited with a lit piece of cord, known as a "match", which is rotated forward by the trigger. After each firing, the match usually had to be relit.

A wheellock is a firearm which comes before the flintlock in history. It has an operation much like the other locks, except that there is a wheel (much like that seen on cigarette lighters) which repeatedly strikes flint to ignite the powder. It was an innovation over the matchlock, but was not widely adopted. (See related: snaphance)

The flintlock was a major innovation in firearms design. The spark used to ignite the gunpowder was no longer generated by a complex wheellock, but rather by a piece of flint set in the jaws of the hammer, which when released by the trigger struck a piece of steel (the frizzen) to create the sparks. The hammer had to be reset after each firing and the flint replaced periodically due to wear from striking the frizzen. See also flintlock mechanism.

Percussive caps operate under the same basic principle as the flintlock, but are more compact and sophisticated. They are also safer, and contain the entire gunpowder explosion inside the weapon. All modern-made muzzleloaders (except those built around the flintlock or earlier types of ignition) use percussive caps. See also caplock mechanism.

A major innovation in firearms came when ammunition, previously coming as separate bullets and powder, was consolidated into a metal cartridge that consolidated the percussive cap, the powder, and the bullet into one package. Cartridges continue to this day to be the standard in modern firearms. In cartridge-firing firearms, a hammer or firing pin strikes the cartridge's percussive cap, igniting the gunpowder. The percussive cap is placed at the rear of the cartridge, either on the rim (in which case it is a rimfire cartridge to be fired in a rimfire weapon) or at the center of the back of the cartridge (in which case it is a centerfire cartridge). As a general rule, centerfire cartridges are more powerful.

Caseless cartridges have been invented: these hold the cartridge together with paper or some other substance that's destroyed when the weapon is fired, eliminating the problem of brass casings ejecting and littering the ground. They are still largely in development.

Most firearms are capable of loading cartridges directly into the breech. Some additionally or exclusively load from a magazine or clip that holds multiple rounds. Generally, a magazine is a box or cylinder that is designed to be reusable while a clip is literally a clip to hold the rounds together that might not even be loadable. Some clips, such as the clip for the M1 Garand, are loadable and reusable, but they do not classify as magazines because they are not necessarily designed to be, and because they are clip-shaped, and not shaped like a box or cylinder.

A firearm that can load multiple rounds as the weapon is re-cocked is considered a repeating weapon. A firearm that automatically recocks and reloads the next round after each shot is considered a semi-automatic weapon. An automatic (or "fully automatic") weapon is one that automatically recocks, reloads, and refires as long as the trigger is held down. Many modern firearms have a selective-fire option, where the weapon can be set with a switch to semiautomatic or automatic firing modes. This feature can be altered--with modern variants of the M16 full automatic fire is not available, but a three-round burst is, while on nearly all selective-fire firearms, the safety (a mechanical device that blocks the trigger) is usually built into the selective-fire switch.

The first automatic weapons were machine guns, usually designed similar to a Gatling gun. Within time, the automatic firing mechanism could be miniaturized. Automatic rifles such as the Browning Automatic Rifle or BAR were in common use during the early part of the 20th century while automatic weapons that fired pistol rounds, known as submachine guns, also became popular during this period.

Submachine guns (such as the infamous Thompson Submachine Gun) were originally the approximate size of carbines. Later on, submachine guns were miniaturized. The most popular submachine gun in the world is currently the Heckler und Koch MP5. However, it is designated as a "machine pistol" by its manufacturer (MP5 stands for Machine Pistol 5), a designation some reserve to firearms such as the MAC-10, which are the size and shape of pistols, unlike the MP5.

However, Nazi Germany developed what was to become the most widespread class of automatic weapon: the assault rifle. The assault rifle was usually smaller than a traditional rifle, but had the capability of fully automatic fire. Soviet engineer Mikhail Kalashnikov quickly adapted the concept to the AK-47, which became the world's most popular assault rifle. The United States reacted differently. John Garand, the inventor of the rifle used by the US military during World War II, the M1 Garand, adapted the design to produce the M14. The significant recoil of the weapon when fired in full automatic mode was seen as a problem, and it was replaced with a more traditional assault rifle in Eugene Stoner's M16.

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