rsync is a program that syncronizes files between two sources. The two sources may be local files on a single computer or one source may be on a remote computer. When operating locally rsync communicates via filesystem calls. When operating between a local machine and a remote host, rsync communicates either via its own unencrypted protocol on port 873 or via an rsh/ssh tunnel.
The original implementation of rsync is a unix command line program that does not work on windows without cygwin.
Alternative and derivative projects
A binary diff program based on the rsync algorithm.
A reimplementation of rsync in windows.
PocketPC port. I will probably write more about this soon.
A cross-platform, bi-direction replication tool. Features GUI or non-GUI operation.
Windows file/folder replication... now standard with Vista.