AirPlay is Apple's technology for streaming video and audio between iOS devices. It is an evolution of AirTunes that was limited to streaming audio from iTunes to Airport Express base stations.
Recent attempts to reverse engineer the protocol suggest that it is based on HTTP live streaming, while audio-only content uses the old AirTunes protocol.
AirPlay was first introduced in iOS 4.2. Early developer builds had the feature integrated in all apps using the standard video controller, but in the final release it was limited to the iPod (Video on iPod touch and iPad) and YouTube apps. However, it is possible to enable AirPlay in 3rd party apps on jailbroken devices .
Currently the only supported target for video/photo streaming is 2nd gen Apple TV, while other companies are expected to release compatible products in the future.
Mac users can download AirPlayer by Erica Sadun that can receive video from an iOS device . It is quite buggy at the moment, does not support audio, DRM-protected video, or iTunes as a source, but it's still an important achievement as the first independent implementation of AirPlay.