"Joomla is an award-winning content management system (CMS), which enables you to build Web sites and powerful online applications. Many aspects, including its ease-of-use and extensibility, have made Joomla the most popular Web site software available. Best of all, Joomla is an open source solution that is freely available to everyone."
Joomla! is one of the most popular open source content management systems and one of the most powerful. It's build on Joomla Framework making it an extensible platform to build websites with.