Shiv is a core rogue ability learned at level 74. In addition to dealing damage, it is used to instantly apply a more concentrated form of the rogue's current active non-lethalpoison and to dispel an Enrage effect.
Though it is not explicitly mentioned in the tooltip, it has been confirmed many times that Shiv cannot be dodged. It may also be unable to be parried or blocked. This makes it useful in PVP situations when your opponent is low on life and uses a temporary avoidance boost (such as Evasion).