From Joomla! Documentation
A CMS is a system for publishing, editing and changing content displayed on a web site's web pages. These types of web pages are commonly referred to as dynamic instead of static web pages because the HTML is assembled and served to the end user only when requested. Dynamic web pages are generated by using some type of built-in functionality for managing and displaying the HTML content items as well as a variety of other data types, such as menus, contacts, and web links.
The Joomla! CMS is a PHP-based system for creating dynamic web pages. Joomla!'s flexible and extensible structure allows functionality to be added using extensions Changing the display (appearance and layout) of the content to the 'end-user' (someone viewing a web site) is achieved through the use of templates and modules.
The home of the Joomla! CMS project is Joomla.org, which contains extensive end-user information. The joomla/joomla-cms GitHub repository contains the source code, while the bug tracker and this documentation wiki are operated separately.