Unlike other video game genres, survival horror relies first and foremost on atmosphere and building tension. These games aren't about defeating hordes of enemies, nor are they about high scores; instead they're about enduring, persevering, managing resources and overcoming terrifying odds.
Often, you'll find yourself exploring an unnerving, claustrophobic environment, armed with nothing but a flashlight and a few dwindling bullets for your already ineffective handgun, not knowing what to expect. It's about the possibility that something might be around the next corner, or hiding in the closet; it's about deceiving the player into thinking that anything is possible, keeping them on edge and making them fear the unknown.
Unfortunately, far too many games now just throw you into a vaguely eerie environment, hand you a map and some kind of objective, and expect you to have a worthwhile time. For every Dead Space and Silent Hill 2, there's a Resident Evil 6, a Silent Hill: Homecoming and an Aliens: Colonial Marines.
For every resounding success there have been about a dozen failures, but there are some that have stood out from the pack and reminded us why we love the survival horror genre in the first place.