From the Back of the Book:
Out of the Far Realm they came: creatures of horror and madness. As the gods rallied against the incursion, brave heroes arose with newfound powers of their own. Together they stood against this threat from beyond, just as they stand today.
This companion to the Player's Handbook (D&D 4e) and Player's Handbook 2 core rulebooks introduces the psionic power source-powers of the will brought to bear against evil's madness. Player's Handbook 3 includes 4 classes tied to the psionic power source: the ardent, the battlemind, the monk, and the psion. It also presents a new divine class (the runepriest) and a new primal class (the seeker).
This essential book for Dungeons & Dragons players contains other exciting options: new races, powers (including skill powers for all classes), feats, paragon paths, epic destinies, equipment, superior implements, and rules for hybrid characters.