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.
