Drupal is a free open source framework and Content Management System (CMS) created with the programming language PHP. Among other Content Management systems, it renders total control to the system administrator, empowering him in creating and organizing content, customizing the web presentation, and automating admin tasks.
Drupal has the inbuilt facility by-design, 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". It would be worth pointing out that 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 PHP and MySQL.