From The Apple Wiki

FairPlay is Apple's propietary DRM used to encrypt media purchased from the iTunes Store. FairPlay is built into the QuickTime multimedia software and used by the iPhone, iPod, iPad, Apple TV, iTunes, and iTunes Store and the App Store. Any protected song or other form of media purchased from the iTunes Store with iTunes is encoded with FairPlay. FairPlay digitally encrypts AAC audio files and prevents users from playing these files on unauthorized computers.