Perhaps the best way to think about Flex and Drupal is to view Flex as a specific - and very powerful - Javascript library for easily creating feature rich User Interfaces on top of the Drupal CMS backend / web app platform.

Other Javascript libraries can be used with Drupal.

The default JS library that comes with Drupal is jQuery.

How does Drupal compare with other Content Management Systems, such as Joomla and Django? Also, what is the best language / framework for creating a CMS?