"WordPress is a free and open source blogging tool and a dynamic content management system (CMS) based on PHP and MySQL. It has many features including a plug-in architecture and a template system" (wiki)

You can upload different themes, customize it exactly how you want - so it's useful for alot of things and there are loads of really cool themes out there
Im not a pro with wordpress as I've only used it a few times but i must admit, it's cool