priest specsFollow

In a nutshell:

Shadow is used for damage and utility. Although in the past it was seen primarily as a leveling tree, the shadowpriest has become a high-demand character in end-game content due to the combination of damage output and mana leeching with Vampiric Touch.

Holy is the primary healing tree. There are a few damage talents here to make soloing less painful, but the focus of the tree is boosting your healing throughput.

Discipline is the tree with no identity. While the first 14 talent points in Discipline are nigh-mandatory for all priests, after that many people become confused as to the tree's focus. There are an abundance of mana talents, some dirty PvP tricks, and a useful raid buff. Typically the only people who go really deep in Discipline are PvPers and people who have specific purposes in mind (like me).