Windows Media Audio 9
This is Microsoft's latest media codec suite. It consists of WMA Standard v1 (0x160), WMA Standard v2 (0x161), WMA Pro (0x162), WMA Lossless (0x163), WMA Voice (0xA).
Initially, WMA was developed by VoxWare and had the name "VoxWare Audio Codec", somewhere on the line Microsoft bought it and continued to develop it to the form it has today.
This is what Microsoft says about their codecs: http://www.microsoft.com/windows/windowsmedia/9series/codecs/audio.aspx
WMA Standard v1 This is the first version of their codec. Originally this came from some other company.
WMA Standard v2 This is the kind of files that is most popular. The current encoder produces this.
WMA Pro The stuff that should have been in WMA std got into Pro. Eg, better sound, more channels etc.
WMA Lossless Not much is known about the lossless codec. It performs about the same as all other lossless codecs.