Advanced Systems Format
Advanced Streaming Format (or ASF, later renamed into Advanced Systems Format) is Microsoft's proprietary digital audio/digital video container format, especially meant for streaming media. ASF is part of the Windows Media framework.
The format does not specify how the video or audio should be encoded, but instead just specifies the structure of the video/audio stream. What this means is that ASF files can be encoded with basically any audio/video codec and still would be in ASF format. This is similar to the function performed by the QuickTime, AVI, or Ogg formats.
The ASF format is based on serialized objects which are essentially byte-sequences identified by a GUID marker.
The most common filetypes contained within an ASF file are Windows Media Audio (WMA) and Windows Media Video (WMV).
ASF files can also contain objects representing metadata, such as the artist, title, album and genre for an audio track, or the director of a video track, much like the ID3 tags of MP3 files.
Files containing only WMA audio can be named using a .wma extension, and files of only audio and video content may have the extension .wmv. Both may use the .asf extension if desired.
The format is often confused with Microsoft's own implementation of MPEG-4 video format (Windows Media Video), because most ASF streams are encoded using this codec.
The ASF container structure is patented in the United States (United States Patent 6,041,345 Levi, et al. March 21, 2000) by Microsoft. They have invoked this patent against VirtualDub, a free video conversion program. Interestingly, Apple's iTunes software now has the capability to convert WMA files to AAC [1].