From Open Source Matters:
Joomla! is a powerful Open Source Content Management System for building professional web sites easily. It is often the system of choice for small business or home users who want a professional looking site that's simple to deploy and use. It can deliver a robust enterprise-level Web site as well, empowered by endless extensibility for your bespoke publishing needs. Joomla! is different from the normal models for content management software. For a start, it's not complicated. Joomla! has been developed for everybody, and anybody can develop it further. Similarly to the Firefox browser, the core system can be extended via easily installable add-ons authored by the developer community.