What is Joomla?

If you really want to create your own website with full capabilities and also easy to manage and also easy to develop, you can choose Joomla as the most popular free content management system to help you create your own site, develop and mantain it.
This easiness including any functionality you can think of, you can found the functionality add-in you need scattered around the internet for freely or you can even write or create your own add-in functionality that you need.

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

Joomla™ is a free content management system used to create websites. Joomla™ allows web designers to construct sites by combining modules, and requires no knowledge of programming to use. Joomla™'s framework is written in PHP, as are the modules used to customize websites. There is a wide variety of modules available, including RSS feeds, polls, and language internationalization, which make it easy for Joomla™ users to publish content to the web, even with minimal web design experience. Joomla™ can also be used to publish information on intranets, making it a useful organizational tool.
http://www.wisegeek.com/what-is-joomla.htm