Joomla Components are the core element that adds functionality to your Joomla site. Joomla components add functionality and features to your Joomla site from simple guestbook component up to ecommerce component such as Virtuemart. Joomla core components include Content, Banners, Contact, News Feeds, Polls and Web Links. There are other 3rdparty Joomla components developed by Joomla community members. There are lots of Joomla components out there, some of those are free and others are not depending on the developer of the component.
Joomla Core Components:
Banners components are used to display image usually intended for advertisement.
Contacts components provide a contact form and information to your site and allow the user to manage a Contacts directory with regard to the Contacts within the web site.
News Feeds components provide a way to display information or articles from external websites on your site to be linked to for further reading.
The Joomla! Poll Manager allows you to create polls using the multiple choice format on any of your Web site pages.
The search component provides search functionality for the information contained in your website.
Weblinks is a component that provides a structured way to organize external links and present them in a visually attractive, consistent and informative way.