Not all packages for Ubuntu is available through its official repositories. Many interesting applications are available from other repositories. Some applications are available from their developers directly in the form of Personal Package Archives (PPA), which are also shared as repositories. These packages can be installed and updated using the apt tools, if their repository is added to Ubuntu.
To add any repository, for example a PPA repository named
ppa:joe/foobar, to Ubuntu:
$ sudo add-apt-repository ppa:joe/foobar $ sudo apt-get update
Always remember to update after adding a repository. Else the package listing from the newly added repository will not be available and so packages from it cannot be found or installed.
Tried with: Ubuntu 12.04.2 LTS