Drupal is an open source CMS framework written in PHP. Its flexible framework structure allows developers to easily attach customized addons.
Drupal is a highly configurable open source Content Management System (CMS), and Web application framework. Questions here can often be answered in a programmatic manner, or through the use of modules and configurations.
The latest version with official releases is Drupal 7 (released on January 5, 2011). Drupal 6 (released on February 13, 2008) is still largely used; Drupal 8 is currently under development, and it is loosely planned for its first official release around 2016. For more details about it, refer to Drupal 8 feature freeze: December 1st, 2012.
Drupal Answers is a dedicated Stack Exchange site for Drupal. You are invited to use it for any questions specific to Drupal which are outside the scope of this StackExchange site. Another site is StackOverflow, with questions using tags such as Drupal-6, Drupal-7, Drupal-8 or just Drupal.