Information and downloads for rsync are available at
http://rsync.samba.org.

rsync uses the "rsync algorithm" which provides a very fast method for
bringing remote files into sync. It does this by sending just the
differences in the files across the link, without requiring that both
sets of files are present at one of the ends of the link beforehand.  At
first glance this may seem impossible because the calculation of diffs
between two files normally requires local access to both files.  A
technical report describing the rsync algorithm is included with this
package.