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).If the maintainer does not tell you how to add the key then you need to find the "key hash" of the repository in order to look up the key on a public key server.Once you know the key hash, the key can be retrieved using the command: Ubuntu Software Center is the GUI-based method to add or remove applications.Once you select a server it remains the selected server until you change it.Note: The fastest server now may not be the fastest server at another time.GUI-based repository management is normally accomplished via "Software Sources".This interface can be accessed via several methods.
While the repository list is updating the Progress icon is displayed in the top bar of Ubuntu Software Center. This will display a repository list in the window above the button.
Packages in PPAs do not undergo the same process of validation as packages in the main repositories.
PPAs are a low-security alternative to the main repositories, so the user will be installing software at their own risk.
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.