Introduction for anti-virus signature creation

Have you ever wondered how anti-virus signatures are created?

In his article An Intro to Creating Anti-Virus Signatures the author describes three types of signature detections:

1. hash signatures
2. byte signatures
3. heuristics

Though the article focusses primarily on Microsoft portable executable files like .exe, .dll and .sys, it would be possible to analyze pdf , .ppt files if one would do that.

The article is well detailed and a good start to understand how anti-virus/anti-malware signatures are built.