Survival horror is a video game genre in which the player has to survive an onslaught of opponents, often undead or otherwise supernatural, typically in claustrophobic environments in a third-person perspective. Horror movie elements are used liberally. The player is typically armed, but not nearly as well-armed as the player in a first-person shooter. The player's goal is generally to escape from an isolated house or town that is inhabited mostly by zombies and/or monsters through fighting and puzzle solving. Isolation is generally one of the most recurrent themes within this genre.

Survival horror is possibly the only video game genre that is defined as much by theme as gameplay style. Though most commonly associated with the Resident Evil mechanics described above, games where the player is made to feel underpowered, generally fighting alone for the bulk of the game, with limited supplies (especially ammunition and health - hence "survival"). They also deal with dark, violent and supernatural themes typical of horror movies. This helps to define the genre more than any specific gameplay mechanic.

One detail a player may notice is the character always has a reason for being there, and more often than not the character does not originate from the said area. This makes as if the character has no real association with the area, or does not know the area. This way the character seemingly doesn't know his way around. Perfect examples can be found in any game, such as the Silent Hill series, the Resident Evil series (except RE 3 and the Outbreak series), and even the Doom series, although the list is not limited to these.

Because of this, the genre is also home to beat 'em ups (Nightmare Creatures), role playing games (Sweet Home), first-person shooters (Call of Cthulhu: Dark Corners of the Earth, Condemned, Hungry Ghosts). Other games such as the Doom and Blood Omen : Legacy of Kain series include horror elements, but give the player a sense of being very powerful. Such games are not typically considered survival horror because horror is not the defining game play characteristic. However, the subjective nature of such discrepancies can make survival horror a difficult genre to classify.

source : http://en.wikipedia.org