The strength in a warrior's shield comes from within. — "Bonewall" Simms (TCG)
Shield Wall is a core warrior ability learned at level 48. It is a defensive self-buff that costs no Rage. It has a 5 minute cooldown. When activated it absorbs 40% of all incoming damage for 12 seconds. It does require a shield to be equipped.
Shield Wall falls into Panic button territory, and is best saved for when you just have to survive a little bit longer. Examples include an OOM healer, just a few adds too many, an enraged end-instance/Raid Boss, too many stacks of a debuff that reduces healing received, or a Battle Ground flag run.
Shield Wall is the most powerful form of damage reduction in the game, and is extremely effective at keeping the warrior alive. In addition, shield wall does not drop aggro, so it allows a warrior to continue tanking, unlike Blessing of Protection.
For more day-to-day panic, there may be less extreme measures available: [Hamstring]ing or [Piercing Howl] (talent) before running, potions, using the much faster cooldown [Shield Block] (Protection specialization) while you finish it off, or even just dying and being raised again by the group healer if it doesn't look like a total wipe is hinged on your survival. Bear in mind that it isn't an invulnerability shield, and even 40% damage from a ?? Elite can still hurt plenty.
/Patch 3.0.2 (14-Oct-2008): Shield Wall now reduces all damage taken by 50% for 12 seconds, cooldown reduced to 5 minutes.
Patch 1.7.0 (13-Sep-2005): Shield Wall will no longer be canceled if you switch stances while the effect is active.