What is this Drupal thing?

Drupal is an open source content management system (CMS) written in PHP by a community of volunteers, originally released in 2001, now in its seventh major release, used by an enormous number of individuals and companies of all sizes across a variety of industries. A few sites running Drupal you might recognize include The Economist, The Grammys, Rutgers University, Eric Clapton, Amnesty International, The US House of Representatives, and The White House.