From Wikipedia (as of April 7, 2015):
Drupal (pronounced /ˈdruːpəl/) is a 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 to large corporate and political sites. In October 2009, the administration of U.S. president Barack Obama adopted Drupal for the official Whitehouse.gov website.
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 (known as contrib modules) created and contributed to the project by open source community members. 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 website installation and administration.
Drupal can run on any computing platform that supports both a web server capable of running PHP version 4.3.5+ (including Apache, IIS, Lighttpd, and nginx) and a database (such as MySQL or PostgreSQL) to store content and settings.