Drupal is an open source content management platform. Equipped with a powerful blend of features, Drupal can support a variety of websites ranging from personal weblogs to large community-driven websites. Drupal is written in PHP and supports various SQL
free, modulated and very good supported
I would be careful when first learning drupal as its a huge system and many ways to trip up but once you learn it then you are able to build great web sites and applications uising its set of api's and extending it via modules
Drupal is the most complete and flexible open-source CMS. And since it's PHP/MySQL, it can easily be installed about anywhere. From a simple personal blog to a full-featured company CMS or e-commerce platform, Drupal rocks!