Source is http://best-damn-codec.blogspot.com/...at-is-wmv.html
WMV is a video encoding/decoding file format developed by Microsoft. In most cases, it uses the .wmv file extension. Comparied with other video file formats in the same visual quality level, WMV file size can be fairly small, so it's suitable for streaming via the Internet. WMV is an i***uential video file format as it's widely used in both PCs and Mac machines for local playback.

WMV is not developed based solely on Microsoft's own codecs, though early WMV 7 and WMV 8 are considered to be Microsoft's proprietary codecs. In 2003, Microsoft drafted a video codec specification based on its WMV 9 codec and submitted it to SMPTE for standardization. The standard was officially approved in March 2006 as SMPTE 421M, better known as VC-1, thus WMV 9 is not proprietary to Microsoft.

An ordinary WMV contains audio data and video data. Its audio data is encoded with Windows Media Audio, while the video data is encoded with Windows Media Video. In most cases, WMV file with the .wmv file extension is packed in a ASF (Advanced Systems Format) container file encoded with Windows Media Video codecs. WMV can also be encapsulated in AVI or MKV container file. When WMV is packed in an AVI file, its file extension is .avi. If WMV is packed in a MKV file, its file extension is .mkv.

WMV-HD

WMV-HD is one of the codecs of WMV. It's a video compression file also developed by Microsoft and mostly uses the .wmv file extension. HD here stands for high definition, and WMV-HD file's resolution is higher than regular WMV file's.