Audio Windows Media (WMA)

WMA is an audio compression proprietary format created by Microsoft for its operating system.
After Microsoft began dealing in online music, it developed this format and WMA now contains two codecs (one lossy, one lossless) for a copyright system.

Also, a WMA archive can be contained in an ASF (Advanced Systems Format) archive which contains multimedia archives (WMA and WMV) and proprietary metadata, created specifically for streaming by Microsoft.

In terms of licensing, this is one of the worst proprietary audio formats because the the licence integrated in one archive is managed and controlled internally by Microsoft which can change it as it pleases, while remaining accountable to no one. For instance, if you buy a WMA music file and the licence allows you to listen to the file on any digital platform and to masterize it, Microsoft can immediately modify this licence and you may be forced to pay a new licence if you wish to be able to keep listening to the file in question.

Open-source alternatives to this format abound: OGG, MPC, FLAC.