It is primarily designed to backup files by copying them to a backup directory. I generally refer to this as an NFS backup because the idea is to use a network mounted filesystem for your backup directory so that you have a copy on another machine in case of hardware failure or software error on the local machine. Although, the backup directory can just as well reside on the local host, preferably on a separate hard drive.
Rbu is the successor to bu and is being completely re-written in Perl. It uses rsync to transfer the data and, unlike bu, can backup to remote destinations (e.g. hostname:/backup/dir) as well as to local file systems.
Once it has all or most of bu's original features integrated in, the plan is to rename it to bu with a new version and drop the original bu from the package. In the mean time, they are capable of working along side one another and sharing the same bu configuration file.