Joomla is a free, open-source Content Management System (CMS) for building websites and online applications. It is popular for its flexibility, extensibility, and built-in features, allowing users to create various types of websites, from small blogs to large e-commerce sites and portals. Key aspects of Joomla include a user-friendly interface, thousands of templates and extensions for customization, robust security, and multilingual support. It is built using PHP and a MySQL database, and is developed by a community of volunteers.