Joomla is a free and open source CMS or content management system for publishing content on the World Wide Web and intranets. It comprises a model–view–controller (MVC) Web application framework that can also be used independently.

Joomla is written in PHP, stores data in a MySQL database and includes features such as page caching, RSS feeds, printable versions of pages, news flashes, blogs, polls, search, and support for language internationalization.

Within its first year of release, Joomla was downloaded 2.5 million times. Over 5,000 free and commercial plug-ins are available for it.

Joomla was the result of a fork of Mambo on August 17, 2005. At that time, the Mambo name was trademarked by Miro International Pvt Ltd, who formed a non-profit foundation with the stated purpose to fund the project and protect it from lawsuits. The Joomla development team claimed that many of the provisions of the foundation structure went against previous agreements made by the elected Mambo Steering Committee, lacked the necessary consultation with key stake-holders and included provisions that violated core open source values.

The Joomla development team created a web site called OpenSourceMatters.org to distribute information to users, developers, web designers and the community in general. A letter was written by the project leader which appeared on the announcements section of the public forum at mamboserver.com.

A little more than one thousand people had joined the opensourcematters.org web site within a day, most posting words of encouragement and support, and the web site received the slashdot effect as a result. Miro CEO Peter Lamont gave a public response to the development team in an article titled "The Mambo Open Source Controversy - 20 Questions With Miro". This event created controversy within the free software community about the definition of "open source". Forums at many other open source projects were active with postings for and against the actions of both sides.

In the two weeks following Eddie's announcement, teams were re-organized, and the community continued to grow. Eben Moglen and the Software Freedom Law Center (SFLC) assisted the Joomla! core team beginning in August 2005, as indicated by Moglen's blog entry from that date and a related OSM announcement. The SFLC continue to provide legal guidance to the Joomla! project.

On August 18, 2005, Andrew Eddie called for community input on suggested names for the project. The core team indicated that it would make the final decision for the project name based on community input. The core team eventually chose a name that was not on the list of suggested names provided by the community.

On September 1, 2005 the new name, “Joomla!,” was announced. It is the Romanized spelling of the Swahili word jumla meaning “all together” or “as a whole.”

On September 6, 2005, the development team called for logo submissions from the community, invited the community to vote on the logo preferred, and announced the community's decision on September 22, 2005. Following the logo selection, brand guidelines, a brand manual, and a set of logo resources were then published on October 2, 2005 for the community's use.

Joomla won the Packt Publishing Open Source Content Management System Award in both 2006 and 2007.

On October 27, 2008, PACKT Publishing announced Johan Janssens the "Most Valued Person" (MVP) for his work as one of the lead developers of the 1.5 Joomla! Framework and Architecture. In 2009 Louis Landry received the "Most Valued Person" award for his role as Joomla! architect and development coordinator.