The Psionics Handbook is a sourcebook for the Dungeons & Dragons roleplaying game that contains a multitude of rules and options for integrating psychic powers into the D&D game.
It is one of the most popular of the optional expansion books written for the game. You must purchase/download for free (legal) the core rules for the game, to use this book.
Several editions of the book have been released, each corresponding to a different edition of the game. Currently, version 3.5 is in use. It is called the "Expanded Psionics Handbook", because it contains many new classes.