Sunder Armor is one of the warrior's basic threat generation abilities. It is an instant attack that decreases your opponent's armor by 4% each stack. Sundering generates a high amount of threat, making it especially useful for tanking. Not only will it help you keep your opponent focused on you instead of your teammates, it will also increase the amount of damage you and your party members deal.
As well as the obvious, and significant, armor reduction effect, Sunder Armor is a powerful tool for establishing and holding aggro as a Main Tank. It causes a solid amount of threat, and is not situational, like Revenge, and has no extended cooldown, like Shield Slam, so can be used over and over repeatedly as long as there is enough rage. (Main Tanks usually have plenty of rage, due to the massive beatings they get.) However, an unimproved Sunder Armor remains a less efficient (per rage used) way of generating threat than an unmodified Shield Slam or Revenge.
In Patch 2.3, the warrior 41-point Protection-tree talent Devastate was changed to essentially be a Sunder Armor that also causes damage. A successful Devastate will cause a Sunder Armor effect on the mob, will generate the additional threat that a Sunder Armor generates, and will also cause damage based on the number of Sunder Armor effects on the target and the DPS rating of the warrior's main-hand weapon. This ultimately means that a warrior with the Devastate ability will be able to use it in any situation where the Sunder Armor effect was previously used (although if they choose to use Sunder Armor this remains a distinct ability).
The threat effect of Sunder Armor continues to occur even after the fifth stacked Sunder Armor debuff, so a tank can continue to reapply it after five sunders have stacked – it will still generate the same amount of threat, but will not stack past five debuffs. Reapplying Sunder Armor on a mob with a full stack of Sunder Armor effects will however reset the expiration timer for the Sunder Armor debuff stack.
Sunder armor does not stack with the rogue ability, Expose Armor. Since their armor reduction effect is higher, Sunder Armor will not be usable while Expose Armor is on the target, making protection Warriors unable to build threat from sundering. So, be sure to let rogues in your group know not to use Expose Armor.