Drupal (pronounced /ˈdruː-pʌl/) is a free and open source modular framework and Content Management System (CMS) written in the programming language PHP.It is used as a "back end" system for many different types of Web sites, ranging from small personal blogs to large corporate and political sites.

The standard release of Drupal, known as "Drupal core", contains basic features common to most CMSs. These include the ability to register and maintain individual user accounts, administration menus, RSS-feeds, customizable layout, flexible account privileges, logging, a blogging system, an Internet forum, and options to create a classic "brochureware" Web site or an interactive community Web site.

Drupal was also designed to allow new features and custom behavior to be added by third parties. For this reason, Drupal is sometimes described as a "Content Management Framework". Although Drupal offers a sophisticated programming interface for developers, no programming skills are required for basic web site installation and administration.

http://drupal.org/