File extension .avs in video editing world quite often refers to a script file created using/for AviSynth.
AviSynth is a program that allows manipulating video files via a scripting language, thus being extremely flexible and convenient tool to do very specific modifications to files or batch of files at once. The downside of this is obviously the fact that as it is a scripting language, the learning curve is rather steep for new users to learn how to do things efficiently and to do most of the scripting using good old text editor.
Related software tools
AviSynth (Open source)
AviSynth is an extremely powerful frameserver for Windows that allows using VirtualDub and TMPGEnc plugins as well as tons of its own plugins to be added to the frameserving process.
AVSEdit (Open source)
AVSEdit is the official script editor for AviSynth.