Drupal is a very powerful CMS (Content Management System). It is a framework that takes care of all the background mechanics of a website and allows you to concentrate on content building.
Drupal is open source, so there is a very large community of users that share designs and modules.
There are many online tutorials and demos.
Zappos.com is powered by Drupal.
The downside to any CMS is that the tend to be targeted by link spammers and hackers. Both the good guys and the bad guys have access to the source code. You must keep the code up to date. Don't rely on your host to do it, you need to stay on top of it.
Drupal has a steep learning curve. It can be frustrating to use, until you get used to it. There are many CMS's available. Wordpress, Moveable type, etc
This discussion from
WebmasterWorld may help you decide on using Drupal.