backup

An up-to-date copy of all your files. You make a backup for several reasons:

Insurance against possible hard-disk or file-server failure. Hard disks often fail completely, taking all your work with them. If this failure occurs, you can reload your files and directories from the backup copy. A backup is your insurance against disk failure affecting the thousands or possibly tens of thousands of files you might have on your file server.
Protection against accidental deletion of files or directories. Again, if you mistakenly delete a file or directory, you can retrieve a copy from your last backup.
Protection against the new version of software you are about to install not working to your expectations; make a backup before installing new software.
As an archive at the end of a project, when a person leaves your company, or at the end of a financial period such as year-end close.

Your decision when or how often to make a backup depends on how frequently important data on your system changes. If you rely on certain files always being available on your system, it is crucial that you make regular, consistent backups. Here are some backup tips:

Keep multiple copies; redundancy should be a part of your backup plan.
Test your backups to make sure they are what you think they are before you bring the server into service, and make sure you can reload the information you need.
Store your backups in a secure off-site location; do not leave them right next to the computer (if the computer is damaged by an accident, the backup may be damaged as well).
Replace your backup media on a regular basis.
Consider making incremental backups of critical data at more frequent intervals.

It all comes down to one very simple rule: back up all the files that you cannot afford to lose. Do it now.