If you are taking the back up using your system tools back up functionality
It takes back up using Déjà Dup. Déjà Dup is a simple — yet powerful — backup tool included with Ubuntu. It offers the power of rsync with incremental backups, encryption, scheduling, and support for remote services.
With Déjà Dup, you can quickly revert files to previous versions or restore missing files from a file manager window. It’s a graphical frontend to Duplicity, which itself uses rsync. It offers the power of rsync with a simple interface.
If you check the back up settings, it shows you all details about backup including path.
It asks you for sign into ubuntu one and stored your files in the on line account.
And It should be exactly like this, if you are taking back up in same system, it is not called as back up, it is simply foolishness because when hard disk is crashed or you get similar situation which does not let you access your documents, it does not ask "please take back up first".