What data needs to be backed up? Quite simply as much or as little as you can live without:

Recovering data from a back-up

The most important thing to remember if you have suffered a failure is not to panic. If you have been taking reasonable steps to protect your equipment and have a backup process in place then you should be able to recover any lost files. Take a methodical approach to recovering the data and make sure you are happy that you are restoring the correct version of the files to the correct place. Many people have panicked in these situations and ended up copying old data over newer copies, making the problem worse. It may be an idea to sit down with a trusted friend or colleague and get them to help double check what you are doing. Having written documentation for your backups can significantly ease this process.

Source: Backing up data

The Business IT Guide enables businesses to make the right IT decisions