To quote Drupal themselves:

Drupal is a free software package that allows an individual or a community of users to easily publish, manage and organize a wide variety of content on a website. Tens of thousands of people and organizations have used Drupal to power scores of different web sites, including:

Community web portals
Discussion sites
Corporate web sites
Intranet applications
Personal web sites or blogs
Aficionado sites
E-commerce applications
Resource directories
Social Networking sites

I couldn't have put it better myself. So I didn't. Click here for more information on the Drupal website.