From Wikipedia
Polyphony Digital, Inc. is an internal video game developing company of Sony Computer Entertainment and is part of SCE Worldwide Studios. The studio was originally known as Poly's Entertainment, however after the success of Gran Turismo they were granted greater autonomy and their name changed to Polyphony Digital.
The studio is best known for its Gran Turismo racing game series. Led by Kazunori Yamauchi, Gran Turismo became the most successful racing series for the PlayStation and PlayStation 2. The Gran Turismo series is designed to be a real driving simulator, offering realistic driving physics. In 2006, Polyphony released Tourist Trophy, in an attempt to bring the realism of Gran Turismo to motorcycle racing.