AirPlay is a proprietary service created by Apple that allows iPod, iPhone, and iPad devices, or computers using iTunes, to send music, photos, or video wirelessly to an AirPort Express, Apple TV, or other AirPlay-enabled device like a receiver.
Formally called AirTunes, the AirPlay feature is often found in iPod docks from Bowers&Wilkins, JBL, Marantz, Denon and others.
AirPlay is intensely proprietary, and will not work with other wireless streaming options.
The easiest way to control AirPlay-enabled devices is with the Remote app from Apple, which lets your iPhone/touch/iPad control your iTunes remotely.
Learn more about AirPlay from Apple's official site.