To begin, you might ask: "what is a data backup?" To answer your question, a data backup is the action of copying files and folders for the purpose of being able to restore them in case of data loss. Data loss can be caused by many things, ranging from computer viruses to hardware failures, file corruption to fire, flood, or theft. Even human error can be the cause by accidental deletion of something important.
In general, you should back up any work or data that can't be replaced easily. Some examples are word processing documents, spreadsheets, databases (for financial data), customer data, and personal files such as pictures, music files, emails, etc.
As a rule of thumb, do not backup programs or system folders. Programs will need to be reinstalled before you can run them and system folders can be restored from your operating system install disc. Be sure to save program installation discs, operating system discs, and registration information in a safe place. If you have downloaded any of your applications, you should save the install files whenever possible.