Advanced Streaming Format

Advanced Streaming Format (or ASF) is Microsoft's proprietary audio/video wrapper, especially meant for streaming. It doesn't 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.

Many cases ASF is confused with Microsoft's own implementation of MPEG-4 video format, because most of the ASF streams are encoded using this technology.