The internet is an excellent tool, and no doubt has changed the way most people communicate, unfortunately the internet, email in particular, has created an easy to target medium for the spread of computer viruses, as you have probably heard these viruses can cause absolute chaos to whole networks of computers at a time.
A virus is basically a malicious computer program, the effect of viruses differ, some either modify, delete or steal data and others may give control of your PC over to their creators via the internet, one thing they all have in common is that if you get infected and you don't have antivirus software you might not know you have it until it is too late.
A Worm refers to a virus that can replicate and spread by itself over a network (the internet for instance), these are getting very common and are among the biggest trouble makers on the internet.
A virus/worm can sit on your machine for months (potentially even years) without doing anything and then be triggered by a certain date/time 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 software has never been so great.
Antivirus software is designed to keep your PC free of these computer viruses & worms, it does so by scanning your PC's 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 files being transferred to and from your computer, any antivirus software should at least scan attachments of incoming emails automatically (even if the option can be turned off).
Virus Definition Files
Antivirus software 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 software without updated definition files is useless.
Most good antivirus software 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 internet to the vendors website (server), and then downloading and installing the latest virus patterns.
This is why it is important to purchase antivirus software 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?