The list will contain all the repositories enabled in your system's software sources.To the right will be the packages available via download (white boxes) and currently installed packages (green boxes).CDs contain software from the "Main" and "Restricted" repositories, so if you have no internet connection you can still install software from the CDs.If you have an internet connection you can install software from any Ubuntu repository.If you wish to search one repository only then click the down arrow to the right of All Software and select the repository that you wish to search.Software Sources is accessible from the Ubuntu Software Center via the Edit, Software Sources menu.It is sometimes useful to find a package's repository (the repository that the package was installed from).
Open the software center, then from the Edit menu select "Software Sources".
Make sure that any repositories that you add have been tested and are known to work on Ubuntu systems.
Repositories that are not designed to work with your version of Ubuntu can introduce inconsistencies in your system and might force you to re-install.
In order to add a repository you need its "location" and the "key command" (the command that will add the repository's key to your system).
For an explanation of the format of the "location", see the Editing Repository Details section below.