Backup is a copy of the database contents that can be used in the case of a system breakdown. The aim is that as much of the database as possible can be restored to its state before a system failure. Backups are usually made at regular intervals according to a standard procedure (complete or incremental backup). Reloading the saved data into the file system is called restoring the data.
SAP R/3 2.1 and 2.2 required you to back up the database before archiving data. This safety measure is no longer required.