An introduction to computer audio
A proprietary protocol for streaming audio/video by Apple.
Initially limited to Apple products only.
Apple has licensed the audio-streaming portion of the AirPlay protocol to others.
From 2011 on streaming audio players appears on the market supporting this protocol.
As Apple talks marketing instead of tech it is almost impossible to find any documentation about the capabilities of this protocol or the Apple devices using it.
Some say it is able of 24 bits/96kHz.
Most Apple devices like Airport Express are limited to 16/44.1
ATV probably resamples everything to 16/48.
This utter lack of specifications causes a lot of confusion for the users.
An example: Airplay & downsampling.
A nice overview of AirPlay enabled devices can be found at AirPlay Speakers.
Philips Fidelio SoundSphere Docking speakers with AirPlay