Extension Installer
From Joomla! Documentation
The Joomla! Installer is designed to allow users to install extensions into their website. Joomla! 1.5 natively allows the installation of modules, components, plugins, templates and languages. Joomla! 1.6 aims to expand this by enabling packages and libraries to be installed.
There is also documentation on the capabilities of the shipped Joomla! installer adapters.