rsync

This article was first written in February 2005 for the BeezNest technical
website (http://glasnost.beeznest.org/articles/205).

rsync is a synchronization tool designed with powerful algorithms to only transfer the minimum needed between source and destination.

Quoting the above web page: rsync is an open source utility that provides fast incremental file transfer.

References:

For a basic explanation of the rsync algorithm, go here