Virus Definitions, what they are and how Antivirus Programs use them

Viruses have defining characteristics that identify them from all the other files on your computer. These characteristics can consist of file names, file signatures, or characteristic metadata(PE headers). An antivirus program checks the files on your computer against a database of these characteristics to identify possible virus infections on your computer.

Autoupdating

This is a process that most anti-virus programs use to keep track of current signatures of viruses. The antivirus program will open a network connection to server on the software makers site and get additional signatures or updating the currently held ones to reflect what the antivirus company observes in the field. By performing frequent updates of these definitions antivirus programs are able to check for the latest viruses that infect computers.