Drupal is a piece of software that helps you create and manage content (information) on a website. This falls into a category of software called Content Management Systems (CMS). In reality, it is more of a "framework" to provide specific functionality related to the "pages" you create and manage, to include a consistent look and feel (a theme) as well as additional tools to control and manage the content, or information.
The main website for Drupal is available at http://www.drupal.org. On that website they describe Drupal as "Community Oriented Plumbing," which does fit in with the orientation of a CMS. Part of the power of any CMS is the ability to easily allow anyone (or some small group of people) to create, manage and/or publish content on the World Wide Web (there is a pretty dynamic security model in Drupal to enforce managing content).
Probably a little more than you wanted, but here's the info on Drupal. Some of these links are for me to share with others as well, and this email just is my "historical documentation" in case I have to find it again. <grin>
Here are a few sites that use (or used) Drupal:
http://www.kpsu.org/ (PSU - Portland, OR site)
http://www.foreignexchange.tv/ (Foreign Exchange with Farheed Zakara)
http://www.spreadfirefox.com/ (Pro Firefox Advocacy web site)
http://www.theonion.com/ (Fake news web site)
http://www.linuxgazette.com/ (Linux News website)
http://www.howtoforge.com/ (Linux News website)
http://linuxjournal.com/ (Linux News website)
http://www.bib.kau.se/ (Karlstad University library)
http://jeroencoumans.nl/journal/cool-drupal-sites (Personal website)
Some links to Drupal related sites include:
Other Drupal Info: