Drupal about itself:
"Drupal, an open source content management platform.

Equipped with a powerful blend of features, Drupal supports a variety of websites ranging from personal weblogs to large community-driven websites."
http://drupal.org/

As far as Wikipedia describes:
"Drupal (pronounced /ˈdruːpəl/) is a free and open source modular framework and content management system (CMS) written in the programming language PHP. Drupal, like many modern CMSs, allows the system administrator to organize the content, customize the presentation, automate administrative tasks, and manage site visitors and contributors. Although there is a sophisticated programming interface, most tasks can be accomplished with little or no programming. Drupal is sometimes described as a "web application framework", as its capabilities extend from content management to enabling a wide range of services and transactions.

Drupal runs in many environments, including Windows, Mac OS X, Linux, FreeBSD, and any platform that supports either the Apache (version 1.3+), or IIS (version IIS5+) Web server and the PHP language (version 4.3.3+). Drupal requires a database such as MySQL or PostgreSQL to store content and settings.

(...)

History

Originally written by Dries Buytaert as a bulletin board system, Drupal became an open source project in 2001. Drupal is an English transliteration of the Dutch word “druppel,” which means “drop” (as in “a water droplet”). The name was taken from the now-defunct Drop.org website, whose code slowly evolved into Drupal. Buytaert wanted to call the site “dorp” (Dutch for “village”, referring to its community aspects), but made a typo when checking the domain name and thought it sounded better.

From May 2006 to April 2007, Drupal was downloaded from the Drupal.org website more than 600,000 times A large community now helps develop Drupal."

http://en.wikipedia.org/wiki/Drupal

Personally, I like the Wikipedia description, cause it its using the term Web Application Framework (http://en.wikipedia.org/wiki/Web_application_framework).