Drupal was developed around 2000, as an open source project, by Dries Buytaert, then at the University of Antwerp, in Belgium. Born as a message board for students, in some years grew and become one of the most popular CMS platforms: it was used by Howard Dean, a candidate to the US presidential election in 2002; this contributed to its diffusion and popularity. Drupal is maintained by a community, but many commercial entities support and sell Drupal products, as Acquia, founded by Jay Batson and Dries Buytaert, the Drupal creator.

Around 2017, more than one million of web sites use Drupal; this is a big number but not comparable to the number of Wordpress sites; Drupal is much more structured and professional than the ubiquitous Wordpress, but is more complex and requires more work to setup and maintain. As Wordpress it has themes, addons (modules), an easy user interface, a simplified editor for web pages, different roles for users, an authorization scheme for publishing. etc., all the features that we find in modern CMS. Drupal is very configurable: the management interface has many, many options and expose to the user a rich set of internal features. This confuse the inexperienced administrator: Drupal is considered difficult and made for professionals.
The frequent release of updates, whit a minor release every month, and the lack of an automated update system for the Drupal core, gives an additional burden to the site maintainer.

Version 7, released in 2011 had a great success, but Drupal was restructured with version 8, which is based on the symfony framework, with its template system: twig. This was a big change and most sites continued to use version 7; themes and addons for version 7 must be nearly rewritten to work with version 8 and their migration was slow. The adoption of Drupal 8, in a world dominated by Wordpress, is difficult and in 2017 most Drupal sites still use version 7.

Drupal has a fully configurable and rich structure:

The internal structure of Drupal changed in version 8: most features are now implemented by modules, which can be enabled or disabled. Blocks, comments, fields, contents, taxonomy, views are all managed by modules.
The "core" of Drupal consists of routines for basic functions, a set of modules and some simple themes. Drupal can be extended with modules and themes from independent contributors, which can be downloaded from the Drupal web site. There are thousands of modules and themes, but most are for version 7. When choosing a module is always a good idea to verify if it is well maintained and if compatible with your version of Drupal. Also the dependencies from external libraries can be an issue.

The Drupal core is actively developed: minor releases, with some feature added, are issued twice an year, each month we have a bug fixes and security patches. A Drupal site requires a constant work; updates are not automatically applied as sometimes happens in Wordpress.