Drupal is an open source Content Management System (CMS). It is a widely used, widely supported CMS that can be used on websites of any size.
Drupal is Flexible
Drupal is an extremely flexible CMS. A Drupal site can be as simple or as complex as the project requires. This flexibility doesn't stop once a site is built. Additional features can be easily phased in over time with little or no disruption to the existing site.
Drupal Modules and Themes
Projects in Drupal don't have to start from scratch. There are over 6,500 Drupal 7 modules and over 400 Drupal 7 themes ready to download. Add this onto a system designed from the ground up to be customized, and you have the basis of a great project before you even get started.
Drupal is Scalable
Do you want to start small but aim for the sky? Drupal works well for small sites but can also handle thousands of users and tens of thousands of pages of content. You don't want your next great idea to be limited by your choice of CMS.