Joomla! is a free, open-source PHP-based content management system.
The package is used to power everything from personal home pages and simple school sites, to online magazines, company intranets and extranets, or major corporate websites and portals.
If you're wondering whether Joomla! can produce the sort of site you need, then the best idea is to spend some time browsing their Community Showcase. It's packed with impressive sites of every possible genre and category, and you'll quickly get a feel for what Joomla! can do.
The system is also fairly straightforward to set up, and can be extended to carry out many more specialist functions via more than 7,000 extensions, most of which are also free of charge.