Windows Media Audio
From Hydrogenaudio Knowledgebase
Windows Media Audio (WMA) are Microsoft's compressed audio formats. It's a generic name for several codecs that can be used inside it:
 Windows Media Audio
Also known as WMA Standard or WMA Std, it was created by Microsoft to compete against MP3, that was quickly becoming the de-facto standard format for lossy compression.
Even though Microsoft claims it is able to deliver the same quality as MP3 at half the bitrates, that statement is certainly false. A more realistic number would be same quality at around 25 % smaller bitrates - and that applies to low bitrates only. At 128kbps, it is easily bested by LAME.
WMA Standard is the second most widespread lossy format (only losing to the ubiquitous MP3), mostly thanks to Microsoft's aggressive marketing tactics.
 Windows Media Audio Professional
Since it's backwards incompatible with WMA Std, Microsoft took the opportunity to make a high quality encoder out of it. Meanwhile WMA Std lost even to MP3 in an informal public listening test, WMA Pro was ranked at the top (next to other high quality formats) in a similar test.
These days, Microsoft is pushing the Pro codec for inclusion in the next generation DVD standard as standard audio format.
 Windows Media Audio Lossless
This is the matematically lossless codec in the Windows Media family. It's a welcome addition since it was the first lossless codec backed up by a major player in the multimedia compression field. Compression efficiency-wise, it's on par with WavPack high and between Monkey's Audio fast and normal.
 Windows Media Audio Voice
This codec, among the first ones to be added to the Windows Media portfolio, is VoiceAge's ACELP.net. It delivers very acceptable voice quality at bitrates around 4 and 20 kbps.