Microsoft's take on the music compression format was first introduced in 1999 to ambitious claims that it would provide the same level of quality as MP3 at half the bit rate. This claim has since been dismissed and disproved, but that hasn't stopped WMA from becoming standard on pretty much every digital music player on the market - apart from those sold by Apple, of course.
And yet, as with MP3 and the less-widely supported AAC, WMA is also subject to licensing fees for those developers who want to include it in their software and hardware.
The feature set of WMA is similar, externally at least, to MP3 and with the latest version - 9.2 - you can encode in Windows Media Player 11 using CBR up to a maximum of 192kbps and VBR up to an equivalent of 298kbps.
But like AAC, WMA was designed to remedy MP3's weaknesses and, as a result, implements several similar improvements. WMA improves upon the complex two-stage process used by MP3 to split the audio signal into subbands, simplifying it to a single stage process. It also has 'improved' stereo coding and more flexible data block sizes than AAC, MP3 and Ogg Vorbis, which in theory means superior efficiency and better quality for a given file size.
There's also WMA Pro, which you'll have seen as an option in the Windows Media Player Rip options tab. This aims to optimise sound quality at very low bit rates, and although not widely supported by dedicated digital audio players, smart phones running Windows Mobile can take advantage of it.