rsync is a replacement for the old rcp (remote-copy) command. It can use ssh for encryption and is a very flexible tool, it can copy from local machine to local machine, from local to remote (and vice-versa), and to and from rsync servers.
rsync uses an advanced differencing algorithm, so when to copies or syncs something it will (a) only copy new/changed files and (b) if the files have being changed it will copy the differences between the files (not the entire file). Using this method rsync saves time and bandwidth.
rsync also has advanced exclusion options similar to GNU tar. rsync has a well written manual page, for further information read the rsync documentation online or type:
man rsync
If you wish to visit the rsync site you will find it over here