The repository components are:

Main - Officially supported software.
Restricted - Supported software that is not available under a completely free license.
Universe - Community maintained software, i.e. not officially supported software.
Multiverse - Software that is not free.

Precise-updates - Updates to official packages.
Precise-backports - Current version software from Quantal Quetzal (Precise+1)
Precise-proposed - Proposed updates & changes (bleeding edge stuff).
PPA repositories: Third party repositories