AirPlay is what Apple calls their proprietary audio and video streaming technology. AirPlay audio and video can be sent from any modern iOS device, including iPhone, iPod touch, and iPad, and as well as OS X on the Mac. Media controllers on iOS and in iTunes on the desktop allow only specific video or audio to be sent via AirPlay, while AirPlay mirroring on iOS and OS X allow the entire screen to be sent. Thanks to AirPlay mirroring, Anything from Facebook and Twitter to racing or fighting game can be sent to Apple TV and displayed on a big screen HDTV. Some apps also allow one or more iOS devices to serve as controllers for AirPlay games.
The video receiver portion of AirPlay currently only works with Apple TV as an end-point, but the audio receiver portion has been licensed out to a number of speaker and home entertainment gear manufacturers, and is supported by Apple's own Airport Express base station.