Imagine you have a very important disk you want to mirror, or have a copy of. This could be a remote disk, like on a website, or even a directory.
You could copy and paste the files, at least if they are local disks, or use FTP. Problem solved, right?
Now, imagine you have two disks, both of which have data, but you need them to match. This was the problem I ran into with over 100 GB of data. I solved this in ubuntu using rsync.
To install, use apt-get:
$ sudo apt-get install rsync