what is the difference between rsnapshot and rdiffbackup?

Quoting from http://www.saltycrane.com/blog/2008/02/backup-on-linux-rsnapshot-vs-rdiff/ :

For rsnapshot, all versions of the backup are accessible as plain files. For rdiff-backup, only the current backup is accessible as plain files. Previous versions are stored as rdiff deltas.

also:

For small files, storage size is similar. For large files that change often, such as logfiles, databases, etc., rdiff-backup requires significantly less space for a given number of versions.

The page lists some other differences also.

I Would add, for the sake of completeness that there exists rbackup which is just a wrapper script around rdiff-backupto ease snapshot management, more like rsnapshot's way of managing snapshots (easier to configure).