Backstab is a level 10 Subtletyrogue ability. It is a hard-hitting move that only works with a dagger in the main hand, and deals additional damage if standing behind the target.
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.
If you want to use the Gouge-Backstab combo in the middle of a fight, be sure that your energy is full: combined they use more energy than you have. If you start the combo with less than 100 energy, you'll have to wait to regenerate some more; by that time the effect of Gouge will have worn off.
With the standard energy regeneration rate of 10 per second, it is viable to Gouge-Backstab starting with ~70 energy. The 4 second duration of standard Gouge should give ample time to get into position and regenerate the minimal 60 energy required for backstab. Though this may be technically possible starting with 65 energy, taking into consideration latency and user reaction, 70 is a safe bet.
When grouped, you can get at a mob's back or side fairly easily. However, if you Backstab whenever you can, you are likely to gain aggro, so use with some care. Consider using [Feint] or [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!
Trivia
Backstab is a basic Rogue spell in Hearthstone. The flavor text reads: "It's funny how often yelling "Look over there!" gets your opponent to turn around."
[Gouge] to move behind the mob and then Backstab. This earns two combo points off the bat! At higher levels it also combines well with 
[Cheap Shot] and 
