A virus program is basically a malicious computer "software" that control your system over to their creators via the web, one thing they all have in common is that if you get infected and you don't have antivirus program you might not know you have it until it is too late. A Worm virus refers to a virus that can replicate and spread by itself over a LAN (the web for instance), these are getting very common and are among the biggest trouble makers on the web. A worm can sit on your machine for months ( even years) without doing anything and then be triggered by a certain date to do what it has been designed to do, this is referred to as its payload. As these viruses-worms become ever more advanced, the need for antivirus programs has never been so great. Antivirus Software Antivirus software is designed to keep your system free of these computer viruses & worms, it does so by scanning your file system looking for known viruses, if a virus is found it will inform you and then take steps to remove the virus threat. Good antivirus software will automatically check any data being transferred to and from your system, any antivirus program should at least scan attachments of incoming emails automatically (even if the option can be turned off). Virus Definition Files Antivirus program usually works by checking a file for certain patterns of binary code, the patterns which it uses to identify viruses are stored in what is known as a virus definition file, when a new virus comes out the virus definition file needs to be updated to include the new virus' pattern. The importance of keeping these definition files updated cannot be overstated, basically antivirus without updated definition files is useless. Most good antivirus program will update these files automatically (or at least have the option to do so), the update of the definition files is usually achieved by having the software connect via the web to the vendors URL (server), and then downloading and installing the latest virus patterns. This is why it is important to purchase antivirus program from an established company, imagine you bought antivirus protection and then 6 months later the company went bankrupt, where would you get your virus definition updates from?