An ASF file is a media file stored in the Advanced Systems Format (ASF), a proprietary video and audio container format. It stores audio and video data, along with optional metadata, such as title, author, and copyright bibliographic data. ASF files are developed by Microsoft primarily for streaming media online.

The ASF container format only specifies the structure of how the audio or video is streamed online, which depends on various factors, such as network bandwidth and server performance. The ASF format does not specify the codec used to encode the audio or video stored in the ASF container format. While the .asf file extension may be used for media files saved in the ASF format, the .wma and .wmv extensions are more commonly used. If the ASF container format stores audio, it is typically saved as a Windows Media Audio (.WMA) file. If it stores video, it is typically saved as a Windows Media Video (.WMV) file. Audio and video data stored within these files may be compressed using a variety of video codecs.