Instantly counterattack any enemy that strikes you in melee for 12 sec. Melee attacks made from behind cannot be counterattacked. A maximum of 20 attacks will cause retaliation.
Don't think of it as retaliation. Think of it as instant punishment.
Retaliation is a warrior self-buff. For 12 seconds, any frontal melee attacks will be met with a vicious counterattack, up to a maximum of 20 retaliations.
Retaliation is a self-buff that costs no Rage. It has a 5 minute cooldown. While it is active, the first 20 melee attacks taken from the front will result in an instant counterattack with your weapon. It has a very obvious animation making its usefulness in PvP hinge on opponents' knowledge of the ability as many opponents will simply stop attacking the warrior when they see the spinning blades—this however could be viewed as a defensive advantage.
Tips and tactics
Although it has a cap on how many counterattacks you can make, it is higher than your average mob will trigger in a fight. So, this skill is best used against multiple enemies, particularly if they attack quickly.
Warrior tanks can greatly benefit from Retaliation's multiple target threat. As of Patch 4.2.0, Retaliation no longer has a stance requirement, so there is no longer a need for tanks to stance dance in order to benefit from this ability.
The counterattacks from Retaliation are normal swings from your current weapon. Because of this, Retaliation is most powerful when used with an extremely slow two-handed weapon.
PvP:
Before the implementation of its animation this was one of the warrior's most powerful abilities, however, many opponents will now simply cease to use melee attacks on the target for this ability's duration.
Using Retaliation during crowded, hectic PvP can be a good way of preventing opponents from noticing its use.
When facing a warrior using Retaliation, melee classes may choose to cease to attack, temporarily flee, or use non-melee attacks (such as [Exorcism]) until the buff expires.
Patch 4.2.0 (2011-06-28): Retaliation no longer has a stance requirement.
Patch 3.0.2 (2008-10-14): Retaliation duration reduced to 12 seconds, charges reduced to 20, cooldown reduced to 5 minutes. Retaliation is no longer linked to the [Shield Wall] or [Recklessness] cooldowns.
[Exorcism]) until the buff expires.
Patch 5.0.4 (2012-08-28): Removed. 
[Shield Wall] or 
[Recklessness] cooldowns. 
