Holy Power is a new paladin resource mechanic in Cataclysm, which works similar to combo points like the rogue's combo point. Holy Power can be gained through certain abilities and further improved by talents to be unleashed with other abilities. It decays over time.
Paladins will generally want to build up Holy Power until it shines through with a bright yellow glowing effect. Once Holy Power has been built up, it can be consumed to augment existing abilities. For example, Word of Glory can be used to cast an instant-cast free heal. This gives paladins several options for how to spend their Holy Power, depending on the scenario.
It becomes available for Paladins at level 9, prior to learning Word of Glory.
Crusader Strike (1 charge, Retribution/Protection. The Retribution talented ability Zealotry increases the generation rate to 3 charges per strike for a short duration.)
The Holy talent Tower of Radiance gives a 33/66%/guaranteed chance of the generation of a 1 Holy Power charge upon the use of these abilities on an ally that acts as the paladin's Beacon of Light (i.e., direct heals only):
The Holy talent Blessed Life gives a 50%/guaranteed chance of the generation of a 1 Holy Power charge upon being struck by a melee attack (2 sec internal cooldown).