Understanding Drupal Video

This video is well worth spending US$30 if you want a very thorough introduction and overview of how Drupal works. You can by it from the Lullabot website. 

In Understanding Drupal, the first in The Lullabot Learning Series, the Lullabot team provides an overview of Drupal as a content management system, as a PHP web application framework, and as a developer community. Its documentary-style exploration covers all the terminology and fundamental concepts for both site administrators and developers. If you’ve ever been confused by Drupal or are still trying to wrap your head around the community and platform, then this video is a roadmap to accelerate your journey up the Drupal learning curve.