Apple’s iTunes 4.0 player is able to play music across a local
network or the Internet, using a proprietary protocol. This protocol
not only tells machines how to stream audio content, it provides
access to playlists and track information as well.
The folks at the Digital Audio Access Protocol project are determined
to reverse engineer that protocol so they can use the iTunes protocol
on other operating systems or platforms.

http://daap.sourceforge.net/