Drupal is a Content Management System. Content management systems (we usually just say CMS) are pieces of software that run on a web server and will allow users to add, modify and organize the content of a web site without having to interact with the design of the web site. Drupal is one of the top open source CMS's out there, and I've just started using it for my clients. I love that it empowers my clients to manage their own web sites and gives them the freedom to realize more of the potential of the Internet.
There is an
excellent video that explains and demonstrates Drupal 5.0.