Drupal is an opensource Content Management System (CMS) which is used to build websites, portals, communities and more. Drupal is modular built, flexible and scalable. Modules control functionality and work together like bricks in a wall; each one fits together to create a whole. Themes control some features of the layout and all other design elements. Content is completely separate from both the functionality and the design.
Drupal can be divided into 5 general user groups: Techsupport/Admin (installation of Drupal, installation of Modules and configuration of user permissions), Site Admin/Designer (configuration of Modules, configuration of site layout, design, setup of users, etc.), Users/Contributors (Authenticated users who can add content and make other changes as designated by the Admins) , and general users (unauthenticated visitors who can access the site and add limited content such as comments).
In individual and small sites, one Admin may do all of the work with the only other user group general users (unauthenticated). In large sites, there maybe many layers of functionality per user group.