Drupal is an open source content management system (cms for short) under the gnu gpl. Written in php and sql, it can be downloaded from www.drupal.org.
Drupal can run on many operating systems and platforms, and is able to work with the apache and Microsoft IIS web servers. It is able to use MySQL, PostgreSQL, or mSQL as a database backend.
Drupal includes all the features normally expected from a content management system, including extensive user management, searchable content, poles, templates, and support for the Blogger API. As well, Drupal has many extra features including allowing modules to extend function, friendly URLs, and much more.
Although drupal is a relatively new content management system, it is beginning to gain popularity with webmasters, and is now used on several popular weblogs, comunity discussion websites, and other collaborative websites. As well, several commercial websites have popped up offering hosting for your drupal website.