Drupal is a content management system written in PHP. It stores information in a relational DBMS. You can use Drupal with MySQL, PostgreSQL etc. Drupal is an open source framework distributed under the GNU General Public License. The Drupal project is pushed forward by the Drupal community of passionate developers, designers and other professionals from all over the world. Drupal can be used to make different types of web sites. The standard release of Drupal suits best the following types of web sites.
Drupal functionality is extended by modules, which can be added when necessary. Every module has an interface to customize its functionality. Permissions to the module are set via roles. Currently there are more than 1000 modules, allowing customization of different features and requiring no programming skills. Full set up can be completed with CSS, PHP and AJAX. Whereas every site is complex in its own way, you can completely change themes, roles permissions and modules settings for just a few hours.