On a standard Ubuntu installation, Ubuntu is configured to use the main repository. There are however, other official repositories (or sections on the Ubuntu server) that aren't available right away. There is one called Universe which is the largest one. It's a pool of community-maintained software, but it is not officially supported by Ubuntu. There is also a section called Multiverse which has software under questionable licences. The third section is called Restricted and is a very small pool of software with restricted copyright.
See InstallingKDE for a full explanation.