from the introduction:
World of Cthulhu is an unofficial supplement designed to be used with White Wolf Incorporated's World of Darkness core rule book. World of Cthulhu is an homage to the supernatural horror of Howard Phillips Lovecraft and, as such, draws heavily upon the public domain works written by that author. Finally, World of Cthulhu was also inspired by the Call of Cthulhu roleplaying game published by Chaosium Incorporated.
Finally, note that the World of Cthulhu rules are not (and never will be) a standalone game — you'll need to pick up a copy of the World of Darkness core rule book to make use of them.