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.

What's a content management system (CMS)?
A content management system is software that keeps track of every piece of content on your Web site, much like your local public library keeps track of books and stores them. Content can be simple text, photos, music, video, documents, or just about anything you can think of. A major advantage of using a CMS is that it requires almost no technical skill or knowledge to manage. Since the CMS manages all your content, you don't have to.

Many companies and organizations have requirements that go beyond what is available in the basic Joomla package. In those cases, Joomla's powerful application framework makes it easy for developers to create sophisticated add-ons that extend the power of Joomla into virtually unlimited directions.

The core Joomla framework enables developers to quickly and easily build:

* Inventory control systems
* Data reporting tools
* Application bridges
* Custom product catalogs
* Integrated e-commerce systems
* Complex business directories
* Reservation systems
* Communication tools

Since Joomla is based on PHP and MySQL, you're building powerful applications on an open platform anyone can use, share, and support. To find out more information on leveraging the Joomla framework, visit the Joomla Developer Network.

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