rsync is a clever algorithm for synchronizing files in a way that is bandwidth efficient. It was invented by Andrew Tridgell and Paul Mackerras. It is described here:
http://rsync.samba.org/tech_report/
rsync is also a program that implements the algorithm.
NasBackup [L1 ] is an Open Source Tcl/Tk application that provides a GUI layer on top of rsync.