Shooter games are a subgenre of action game, which often test the player’s speed and reaction time. Because „shooters make up the majority of action games“, it is a fairly wide subgenre. It includes many subgenres that have the commonality of focusing „on the actions of the avatar using some sort of weapon. Usually this weapon is a gun, or some other long-range weapon“. A common resource found in many shooter games is ammunition.

Characteristics of shooters
There are many criteria to determine the type of shooter; listed below are some of the major divisions. Using the following, it is possible to categorize almost all shooters developed.

Perspective
The player usually views the events from behind the eyes of the character (a first-person shooter) or from a camera that follows the character, usually a few feet behind (a third-person shooter). It is also possible for a game to have a fixed camera, especially shooting gallery games and some 2D overhead shooters such as Robotron 2084.

Realism
Games that attempt to emulate life-like ballistics and character damage include tactical shooters. But many shooter games involve varying levels of realism, with some verging on complete fantasy.

Number of characters
While most shooters are played as solo ventures, several offer the players the opportunity to control a squad of characters, usually directly controlling one, and giving orders to computer-controlled allies. Games which feature non-player characters fighting alongside the player, but which are not directly controllable (either by switching player control, or issuing orders to the character) are not considered squad-based games.

Multiplayer
If a shooter game is available online, there are several other sharp divisions it can take. Team games are where players are assigned to one of two (sometimes more, but very infrequently) factions which are competing for some goal. Co-Op games have several players on the same faction playing through either the single player or custom missions against computer-simulated enemies. Individual(often called Deathmatch) has all players competing with each other. Many times a game will offer differing modes which allow players to choose from among these various types.

Focus
This is often an optional way to categorize a shooter, but in some cases it’s needed to help distinguish it. A game may quite often heavily rely on stealth as opposed to direct action. Others might have large horror elements to them. However, the one thing in common with all shooters is that combat with a gun or similar long range/projectile weapon is the primary focus of gameplay itself.

Sub-genres

Shoot ‚em up
Shoot ‚em ups are a specific sub-genre of shooters, wherein the player has limited control over their movement; if done with 2D gameplay, an example being Ikaruga, this means that a moving background continually pushes the player forward, though they may move up and down and left and right around the screen, typically firing straight forward.
Shoot ‚em ups share common gameplay, but are often categorized by viewpoint. This includes fixed shooters on fixed screens, scrolling shooters that mainly scroll in a single direction, top-down shooters where the levels are controlled from an overhead viewpoint, and rail shooters where player movement is automatically guided down a fixed „rail“. This genre also includes „run and gun“ games which emphasize greater maneuvering or even jumping.

Shooting gallery
Often interchangeable with light gun games, although many could also be played using a regular joypad and an on-screen cursor to signify where the bullets are being aimed. When these debuted, they were typically played from a first-person perspective, with enemy fire that occurred anywhere on the screen damaging or killing the player. As they evolved away from the use of light guns, the player came to be represented by an on-screen avatar, usually someone on the bottom of the screen, who could move and avoid enemy attacks while returning fire. These sorts of shooters almost always utilize horizontal scrolling to the right to indicate level progression, with enemies appearing in waves from predestined locations in the background or from the sides.
As light gun games became more prevalent and started to make use of fully 3D backgrounds (such as the Time Crisis or House of the Dead series), these sorts of games fell out of popular production, but many (like NAM-1975) still have their fanbase today.

Light gun shooter
Light gun shooters use a pointing device for computers and a control device for arcade and video games. The first light guns appeared in the 1930s, following the development of light-sensing vacuum tubes. It was not long before the technology began appearing in arcade shooting games, beginning with the Seeburg Ray-O-Lite in 1936. These early light gun games used small targets (usually moving) onto which a light-sensing tube was mounted; the player used a gun (usually a rifle) that emitted a beam of light when the trigger was pulled. If the beam struck the target, a „hit“ was scored. Modern screen-based light guns work on the opposite principle — the sensor is built into the gun itself, and the on-screen target(s) emit light rather than the gun. The first light gun of this type was used on the MIT Whirlwind computer. Like rail shooters, movement is typically limited in light-gun games.
Notable games of this category include Resident Evil: The Umbrella Chronicles, the Virtua Cop, the Time Crisis series and House of the Dead series, along with Duck Hunt for the NES.

First-person shooters
First-person shooter is characterized by an on-screen view that simulates the in-game character’s point of view. Notable examples of the genre include Doom, Quake, Half-Life, Counter-Strike, GoldenEye 007, Battlefield, Unreal, Call of Duty, and Halo.

Third-person shooters
Third-person shooters are characterized by a third-person camera view that fully displays the player character in his/her surroundings. Notable examples of the genre include Rare’s Jet Force Gemini, the Max Payne and Gears of War series.

Tactical shooters
Tactical shooter is a shooter that generally simulates realistic, squad-based or man-to-man skirmishes. Notable examples of the genre include Ubisoft’s Tom Clancy’s Rainbow Six and Ghost Recon series and Bohemia Software’s Operation Flashpoint.

Sources: Wikipedia Modified by Fanterazzi