Even though you may have a decent anti-virus program installed, it is still necessary to backup your personal data, such as documents, photos, music and emails. A lightning strike, power surge or hardware failure could wipe out your personal data with no hope for getting it back.
Backing up your data and files can help you avert this situation. It is simply the act of making a copy of your data and files and then storing the copy in a safe place. If you do this regularly you will then be able to retrieve your data.
This can all be achieved in various ways, either an external hard disk, a USB memory stick, CD/DVD or in the cloud.