Components are the main functional units of Joomla and can be viewed as mini-applications. The content is usually displayed in the centre of the web page is usually created by a component.
Most components have two main parts: an administrator part and a site part. The site part is what is used to render pages of your site when they are requested by your site visitors during normal site operation. The administrator part provides an interface to configure and manage different aspects of the component and is accessible through the Joomla administrator application.
Joomla comes with a number of core components, for example, com_content for articles, com_contact for contacts and com_menus for setting up the website menus and menu items.