Components are the largest and most complex extensions of them all; they can be seen as mini-applications. Most components have two parts: a site part and an administrator part. Every time a Joomla! page is requested, one component is called to render the body of the page. For example, Content (com_content) is the component which handles the display of content; users can view at the frontend of your site and, as an administrator, you can edit the content. Components are the major portion of your page because a component is driven by a menu item and every menu item runs a component.
Examples: Content (com_content), Banners (com_banners), Contacts (com_contact), News Feeds (com_newsfeeds) and Web Links (com_weblinks)