Drupal is free and open source Content Management System (CMS) written in PHP and distributed under the GNU General Public License. It is used as a back-end system for many different types of websites, ranging from small personal blogs to Enterprise 2.0 collaboration and knowledge management uses.

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 within a flexible and rich permission / privilege system, create and manage menus, RSS-feeds, customize page layout, perform logging, and administer the system. As installed, Drupal provides options to create a classic brochureware website, a single- or multi-user blog, an Internet forum, or a community website providing for User-generated content.

Drupal was also designed to allow new features and custom behavior to be added to extend Drupal's core capabilities. This is done via installation of plug-in modules created and contributed to the project by open source community members.

Drupal is also described as a web application framework, as it meets the generally accepted feature requirements for such frameworks.