ASF is a media container used by Microsoft Windows Media Player. It was designed for streaming audio and video files online. The container determines the structure of the data stream but not how the data should be encoded. Usually the files encoded by Microsoft developers have the .wma or .wmv extensions, other files have .asf extension. Such files generally have MIME type that allows sending files via the internet. The main feature of the ASF is the ability to stream playback, that is the ability to play a video or audio directly at the time of downloading data over the network.

WMV is a codec usually used for video compression in ASF format. It is possible to open WMV files both on Mac and Windows with almost any player or convert the format to another one. WMV supports DRM protection that prevent users from copying information. It is the main reason why the format is widely used by companies that sell digital video and audio online. Also the format is well-known among users because WMV files are used on popular Xbox 360 devices. Low bitrate makes such files an ideal solution for HD DVD and Blu-ray discs.