Launchpad is a web application and website that allows users to develope and maintain software. Software is stored in PPAs (Personal Package Archives) which allow users to easily install software that isn't available in the official repositories. In this guide I will describe you how to use such a PPA.
To add a PPA type in a terminal:
sudo add-apt-repository ppa:oneppa/ppa
Afterwards you have to update your software sources so that you can "see" the packages available in the PPA. To do so type:
sudo apt-get update
Disabling a PPA means that you will no longer receive updates from that PPA. This doesn't remove the PPA nor the packages you have installed from it. The advantage is that you can easily reenable it the same way.
To disable a PPA open Ubuntu Software Center > Edit > Software Sources and uncheck the box next to the main and the source code line. The same way you can reenable it.
Removing a PPA means that you only delete the repository from your computer. Any packages you may have installed or updated from that PPA will remain installed.
sudo add-apt-repository --remove ppa:oneppa/ppa
Purging a PPA means that you disable a PPA and moreover you downgrade any packages you have upgraded from that PPA to the latest available version in the official Ubuntu repositories. However be careful, if you have installed a package from a PPA that you purge and which doesn't exist in the official repositories you have to manually uninstall it.
You need a little utility to purge a PPA. Just type:
sudo apt-get install ppa-purge
Now you can purge a PPA by typing:
sudo ppa-purge ppa:oneppa/ppa
Of course you can use Ubuntu Software center in order to add, disable or remove a PPA. However I think that the terminal way is much faster.