WMA

WMA (Windows Media Audio) is an audio format and codec developed by Microsoft and used as the native format for compressed audio to be played in Windows Media Player, as WMV is for video. As a file format, WMA is usually contained within an ASF container format.

WMV does not inherently contain Digital Rights Management (DRM), but the ASF container format was designed with the possibility of DRM in mind, so some of these files are so encumbered.

Links

 * WMA: Wikipedia article
 * Microsoft's Windows Media page
 * Windows Media encryption algorithm description
 * WMA decoder source code