PPA's (Personal Package Archives) contain software that is not offered by default in Ubuntu. PPA's include various software including themes, utilities and standard programs.

1 Software & Updates 

Open the Gnome Launcher and click on Software & Updates. Click on the second tab called Other Software.

2 Add a PPA

Click on Add. Enter the PPA name, these follow the format of ppa:starlabs/ppa

3 Remove a PPA

To remove a PPA, select the PPA and click remove.

Please note: This will only remove the PPA itself, any software installed via this PPA will not be removed.