and what do I use to play one?

 

Windows media player

its a streaming video file

 

Another file extension

To play a video in one of the ways

it was extracted.

Update Windows Always.

axcilla

 

Like the others have pointed out, it's an extension for the type of file created by Microsoft's windows media encoder. It stands for "Advanced Streaming Format". You may also see .asx which is just a pointer file to an .asf file.