The best place to start for a description of Drupal is here on the Drupal.org website. You can also find a number of case studies there to review and to find our more about how others are using the software.
For beginners, Drupal is a complex Swiss-Army knife that is best used when you have run out of options. Unless you have some help, you will need to use a service like Bryght or Dreamhost, to have them install the software for you.
For intermediate users, Drupal is a collection of functionality and features that can be an endless source of enjoyment and frustration, giving you the opportunity to truly create something that meets your web publishing needs. It will take a lot more time than you might think, and in the end you will really know something about your site if you keep at it.
For advanced users, Drupal is the best of class for developers, and usually requires 100+ hours before you can really start coding for it. You are rewarded with a cadre of development partners who will support you and rail on your code, and you can build some amazing sites really, really fast.