There are no sub-classes, per se, in the game. However, players of the same class can differentiate themselves by choosing to specialize in three different trees of talents. A priest, for example, has access to talents in the holy, restoration, and shadow trees. One priest could become a "shadow" priest by wow power leveling specializing in shadow talents, while another could become a "holy" priest by focusing on holy talents. All classes can specialize through talents.