Rsync is a file transfer program for UNIX- or POSIX-based operating systems. It 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. Some features of rsync include:
Rsync was formerly used (prior to the advent of complete CD- or DVD-sized ISO images being made available) to download the Debian GNU/Linux pseudo-image kit to get the ISO image. It worked by downloading the packages from the Debian FTP servers and then using rsync to convert the packages into a burnable ISO image. Rsync tends to work better under UNIX/Linux than under MS-DOS, but oh well.