The Player's Handbook book published for any RPG system intended for the use of the players as opposed to the referee. In most cases the Player's Handbook (commonly abbreviated PHB) will provide the basic rules of the system (and be used by both players and GM) while other materials are provided explicitly for the GM.
In Dungeons & Dragons, the game has traditionally consisted of the PHB; the Dungeon Master's Guide, or DMG; and the Monster (or Monstrous) Manual, or MM. Other games have different naming conventions. In some games, such as Trail of Cthulhu, the PHB is simply a cut-down version of the main book with the GM's sections expunged.