http://www.joomla.org/about-joomla.html

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 basically a CMS i.e. Content Management System, which is used for publishing web contents very easily.