ASF stands for Advanced Systems Format or Active Streaming Format, which is the file format used by Windows Media. An ASF file can store the compressed content of Audio and/or Video. It supports playback with the Windows Media Player (offered the proper codecs installed), streaming with Windows Media Services or appropriate package with Windows Media Rights Manager.

ASF dose not specify the structure for media streams within the file. Instead, it specifies a standardized, extensile file container that is independent on a peculiar operating system or communication protocol, or on a particular method, like HTML or MPEG 4, for constituting the data stream within the file. This function is similar to that of the QuickTime, AVI, or Oqq container formats. There are three parts contained in an ASF file: a Header object including information about the file itself, a Data object including the media streams, and an optional Index object that contributes to enable random access to data in the file.

The format was primarily created by Microsoft Corporation to support streaming media, and initially known as Advanced Steaming Format. However, besides the function of streaming, it also could almost successfully replace RIFF, so it came to be named by Advanced Systems Format.

ASF can store synchronized multimedia data. It allows to delivery data over a great number of networks and protocols meanwhile ensuring fit for local playback. ASF enables state-of-the-art multimedia capabilities, such as extensible media types, component download, scaleable media types, author-specified stream prioritization, multi-language support, extensive bibliographic capabilities, such as documents and content management. The Windows Media SDKs which enables creating and reading Windows Media content including ASF files are widely adopted among Windows software developers.

There are many programs that can be used to open ASF files:
Windows operating system: Microsoft Windows Media Player, VideoLAN VLC media player, Roxio Creator 2010, CyberLink PowerDVD 10, CyberLink PowerProducer 5
Linux operating system: VideoLAN VLC media player
Mac operating system: Microsoft Windows Media Player, VideoLAN VLC media player