1. Rsync uses a quick and reliable algorithm to very quickly bring remote and host files into sync. Rsync is fast because it just sends the differences in the files over the network (instead of sending the complete files). Rsync is often used as a very powerful mirroring process or just as a more capable replacement for the rcp command. A technical report which describes the rsync algorithmis included in this package. Install rsync if you need a powerful mirroring program. From Mandrake 9.0 RPM

2. fast remote file copy program (like rcp) rsync is a program that allows files to be copied to and from remote machines in much the same way as rcp. It has many more options than rcp, and uses the rsync remote-update protocol to greatly speed up file transfers when the destination file already exists. The rsync remote-update protocol allows rsync to transfer just the differences between two sets of files across the network link. From Debian 3.0r0 APT