Priests in the World of Warcraft are a single class that incorporates a wide variety of characters. They are best known for casting spells that call forth the power of the Holy Light, but the priest using these spells in the game mechanics doesn't necessarily have much connection to the Light as such -- rather they have a connection with their own religion which grants them similar effects to those of the Light.
When WoW was being developed, Blizzard realized that night elves and trolls, for instance, would not follow the Light in the same way humans and dwarves do, so they tried to represent a bit of this diversity through race-specific spells. It didn't work out, though -- some were too powerful, while others weren't worth reading about, much less putting on one's action bar. The end result was that they made some of these spells universally available to all priests, and completely removed the rest. Here the lore had to surrender to the game mechanics in order to provide the best game balance.
In roleplaying, however, there is a lot of room for players of different races to behave differently, and draw their powers from totally different sources. Greater Heal, for instance, could come either from the Light or the power of Elune. A Shadowfiend could either be a spawn of the Forgotten Shadow, or a dark trollish voodoo spirit. If you are roleplaying a priest, the only thing that really matters is that your character have some sort of faith or profound belief, which could serve as the source of their divine magical power. A priest's magic revolves around his or her strong beliefs and ideas -- but what those beliefs are is entirely up to you.