Airplay was released on September 1, 2010. It allows for streaming audio and video and related metadata between Apple devices. Its predecessor, AirTunes, was released on June 7, 2004 and was only for audio.
Devices capable of sending Airplay content:
Computers running iTunes (audio only), iOS devices running iOS 4.2 or greater. Computers running OS 10.8 Mountain Lion (scheduled to be released in summer of 2012) are able to send both audio and video.
Devices capable of receiving Airplay content:
Airport Express (audio only), the Apple TV (both audio and video) , and third party speakers (obviously audio only).