rdiff-backup backs up one directory to another, possibly over a
network. The target directory ends up a copy of the source directory,
but extra reverse diffs are stored in a special subdirectory of that
target directory, so you can still recover files lost some time ago.
The idea is to combine the best features of a mirror and an incremental
backup. rdiff-backup also preserves subdirectories, hard links, dev
files, permissions, uid/gid ownership, and modification times. Also,
rdiff-backup can operate in a bandwidth efficient manner over a pipe,
like rsync. Thus you can use rdiff-backup and ssh to securely back a
hard drive up to a remote location, and only the differences will be
transmitted. Finally, rdiff-backup is easy to use and settings have
sensical defaults.