Advanced Streaming Format (ASF) - Microsoft's proprietary digital audio/digital video container format, especially meant for streaming media. ASF is part of the Windows Media framework. ASF content can be in the form of a live stream or an .asf file generated by Windows Media Encoder. ASF content being delivered over a network is called an ASF stream.
http://en.wikipedia.org/wiki/Advanced_Streaming_Format

Advanced Streaming Format file (.asf) - An audio or video file that is formatted in ASF.

ASF Stream Redirector file (.asx) - An ASX metafile used by Microsoft Windows Media Player regarding unicast streams, multicast streams and other supported media.