There are four main types of extensions that you can obtain and install for Joomla.
Some common extensions for Joomla include, backup extensions, forums, commenting systems, event calendars and document management systems.
To find more extensions to increase your website functionality, please visit, http://extensions.joomla.org. Here you will find a over 5500+ extensions available for Joomla. There you will find a huge list of components, modules and plugins.
Templates on the other hand can be found a specialised template shops such as JoomlArt and Rockettheme.