"Drupal" is a content management system (CMS) that is widely used, in active develpment,
and also happens to be very well written. A "content management system"
is a bunch of computer code that makes it easy for you to build and
maintain your website. In this case, it's not a program on your
computer, but on the computer that serves your website. Here's the story about why it's called Drupal.
It's also popular - it just won the Overall 2007 Open Source CMS Award from packt publishing.
There are lots of good reasons to use a content management system, and most of the time I recommend Drupal. Of course, not all sites need that much complexity, or that type of complexity, so I can be ecumenical (or maybe just practical) when necessary.
For example, this site isn't built with Drupal.
To learn more about Drupal: