a utility used to sychronise files between locations (possibly over a network)
rsync is a commandline tool for efficiently copying and backing up data from one location (the source) to another (the destination). It can be used for local backup of files, a backup over a network, synchronising a remote website and many other uses.
While rsync can be used directly from the commandline or from within a script there is also a gui for rsync called grsync for Ubuntu users who are not willing to use the commandline.
See also: grsync