Windows Media Video (.wmv) is a compressed video file format for several proprietary codecs developed by Microsoft documented in
Microsoft KB 288102. The original WMV codec was originally designed for Internet streaming applications. The first version of the codec, WMV 7, was introduced in 1999, and was built upon Microsoft's implementation of MPEG-4 Part 2.
WMV 9 introduced several important features including native support for interlaced video, non-square pixels, and frame interpolation. WMV 9 also introduced a new profile titled Windows Media Video 9 Professional that supports high-resolution video. Through standardization from the Society of Motion Picture and Television Engineers (SMPTE), WMV has gained adoption for physical-delivery formats such as HD DVD and Blu-ray Disc.
For more details see the Wikipedia article:
Windows Media Video