Guarded by the Light is a passive paladin ability learned at level 10 for those with the Protection specialization. It operates more like a personal buff, but is in many ways a mode change. What makes it like a mode is the sub-effect "Your spell power is now equal to 50% of your attack power, and you no longer benefit from other sources of spell power."
Patch changes
β Patch 5.0.1 (Beta): Effects changed to "Increases your total Stamina by 15% and your block chance by 10%. Reduces the chance you will be critically hit by melee attacks by 6%. Word of Glory is no longer on the global cooldown. Your spell power is now equal to 50% of your attack power, and you no longer benefit from other sources of spell power. Grants 6% of your maximum mana every 2 sec." This talent is automatically learned when choosing the Protection specialization at level 10.
Patch 4.2.0 (28-Jun-2011): Guarded By The Light no longer interacts with Holy Shield in any way.
/Patch 4.0.1 (12-Oct-2010): Renamed "Guarded by the Light" from "Touched by the Light". Rank 1 effects changed to "Increases your Word of Glory by 30% when used to heal yourself. Any overhealing will create a protective shield equal to the amount of overhealing that lasts for 6 sec. In addition, your Word of Glory will also cause Holy Shield."