When discussing the major changes that paladins saw in Cataclysm, holy power is always at the top of the list. While hunters received their pseudo-energy resource called focus and death knights had their rune system revamped, holy power was easily the most interesting mechanic that was introduced. There's no way to play a holy paladin efficiently without completely adopting holy power. The two holy power releases are both free and flexible. They only cost holy power to use, and they give us a strong instant heal and the only instant AoE heal in our toolbox.
We receive a relatively fixed amount of holy power every minute, with the biggest contributor being Holy Shock. We can only use Holy Shock about 10 times a minute, although a few of our talents help augment that number. The key concept is that holy power is a fixed resource, which means that deciding between Light of Dawn and Word of Glory becomes an important choice.