Backstab is a rogue ability learned at level 40 for those with the Subtlety specialization. It is a hard-hitting move that is performed from behind the enemy. It deals 210% physical damage while awarding a single combo point. It only works with a dagger in the main hand.
Humanoid mobs will often attempt to flee when they are low on health. When a mob does so, it will also often turn its back to you and keep running even though you are behind it, giving you the chance to perform multiple backstabs before the mob decides to fight you again.
When grouped, you can get at a mob's back fairly easily. However, if you backstab whenever you can, you are likely to gain aggro, so use with some care. Consider using [Vanish] to keep your threat level down.
In PvP, a common method of moving behind another character is to jump forward through them, hold your right mouse button and rotate in mid-air to complete a 180 degree turn. You will land right behind and facing your opponent ready to follow up with a quick backstab!