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