About ASF, WMA, WMV
The Advanced Systems Format (ASF) is the file format used by Windows Media Technologies. Audio and/or Video content compressed with a wide variety of codecs can be stored in an ASF file and played back with the Windows Media Player.
The Advanced Systems Format (ASF) is an extensible file format designed to store synchronized multimedia data. It supports data delivery over a wide variety of networks and protocols while still proving suitable for local playback. ASF supports advanced multimedia capabilities including extensible media types, component download, scaleable media types, author-specified stream prioritization, multiple language support, and extensive bibliographic capabilities, including document and content management.
The Advanced Systems Format file container stores the following in one file: audio, multi-bit-rate video, metadata (such as the file's title and author), and index and script commands (such as URLs and closed captioning). The file container supports files as large as 17 million terabytes.
To ensure that content is associated with compatible players, there are several different file extensions. The file extension used should be:
.WMA for files that include audio compressed with the Windows Media Audio codec, or
WMV for files that include both audio and video compressed with Windows Media Audio and Windows Media Video codecs.
Content compressed with other codecs should be stored in file and use the .ASF extension.
By using separate extensions, users can install multiple players on their system – some which may play only audio, and associate with the .WMA extension, and different players that play both audio and video, and associate with the .WMV extension.
Original text: http://www.xvsonline.com/xfiles/techart.htm