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.

This is a lossless-format (and encoding algorithm) for Apple platforms. M4A is widely used in iPod players and as a component in QuickTime media-players. The m4a codecs decoder is found in open sources – this allows third party users to open files in that format.