The Windows Media video format is a proprietary media format developed by Microsoft. It uses one of several Microsoft codecs to compress audio and video data.
WMV files are encapsulated within Advanced Systems Format (ASF) container format. ASF files that do not use WMV codecs are typically saved with a .ASF extension. Windows Media video files that are saved within AVI or Matroska container formats are saved with .AVI or .MKV extensions respectively.