rdiff-backup / host.net::/target-dirwill backup your root filesystem and all the subdirectories to host.net, making /target-dir a mirror of /, and storing increments in /target-dir/rdiff-backup-data. Assuming, of course, that:
rdiff-backup / email@example.com::/target-dirbut then rdiff-backup wouldn't make the dev files. Also, all the files in /target-dir would be owned by user, because only root can chown. However, uid/gid information would be preserved in a separate file.
In reality, other options would probably be added, for instance to exclude /proc and tmp directories, change permissions if necessary, etc. Here is how I used to run it nightly on my system:
/usr/bin/rdiff-backup --exclude /usr/local/games --exclude /tmp/'*' --exclude /var/tmp/'*' --exclude /mnt --exclude /proc --exclude /sys / /mnt/back