What is Drupal?
Drupal is a content management system (CMS) and a solid foundation with a well-documented API for building modern websites. CMS is the Holy Grail of separating the content from the layout system and gives you an easy way to publish and maintain your site. Drupal is one of world biggest Open Source based CMS. Here are some examples of high profile sites running Drupal:
More information about new and exciting new Drupal based sites is found on Dries homepage.