Joomla! already is a rich featured content management system, but if you’re building a website with Joomla! and you need extra features which aren’t available in Joomla! by default, you can easily extend it with extensions. There are five types of extensions for Joomla!: Components, Modules, Plugins, Templates, and Languages. Each of these extensions handle specific functionality (some built-in features of Joomla! are implemented using extensions).

http://extensions.joomla.org/