Drupal is a free and open source content-management framework written in PHP and distributed under the GNU General Public License, providing a back-end framework for among the largest sites on the internet, ranging from personal blogs to corporate, political, and government sites.
The standard release of Drupal contains basic features common to content-management systems including user account registration and maintenance, menu management, RSS feeds, taxonomy, page layout customization, and system administration. Drupal can serve as a simple Web site, a single or multi-user blog, an internet forum, or a community Web site providing for user-generated content.
Drupal offers modules (equivalent to Joomla! Extensions and WordPress plugins) to enhance the core platform, and has a mature ecosystem of developers offering custom work, support, and more. The original platform with which CiviCRM integrated (2005), Drupal offers tight integration via plugins and can be configured to create powerful workflows within the CiviCRM + Drupal installation.
If you’re familiar with sophisticated content management, or have complex workflows or usage requirements, Drupal may be an ideal fit for your needs.