If you want to backup a complete server, backup consists of many individual steps. You will not only have to backup all files on the server (e.g. by rsync), but you need to backup boot manager and partition data, too. Backups need to be done to another server or FTP storage space. This is far beyond the scope of Plesk. Please consult a Linux expert for assistance on backing up Linux machines.
Most users do not backup their machines, because OS and Plesk can easily be re-installed from images and autoinstallers. Normally it should be sufficient to backup Plesk contents and configuration completely using the Plesk backup manager. In a total disaster recovery scenario the time to reinstall a server and restore a full backup through Plesk is about the same as reinstalling a server from an individual backup of partitions, boot manager, file backups.