Subcategories
Components
Components are larger extensions that produce the major content for your site. Each component has one or more "views" that control how content is displayed. In the Joomla administrator there are additional extensions such as Menus, Redirection, and the extension managers.
Modules
Modules are small blocks of content that can be displayed in positions on a web page. The menus on this site are displayed in modules. The core of Joomla! includes 24 separate modules ranging from login to search to random images. Each module has a name that starts mod_ but when it displays it has a title. In the descriptions in this section, the titles are the same as the names.
Templates give your site its look and feel. They determine layout, colours, typefaces, graphics and other aspects of design that make your site unique. Your installation of Joomla comes prepackaged with three front end templates and two backend templates. 
Plugins are small task oriented extensions that enhance the Joomla! framework. Some are associated with particular extensions and others, such as editors, are used across all of Joomla. Most beginning users do not need to change any of the plugins that install with Joomla.