(Text borrowed from Drupal.com, the Drupal-targeted and -related company)
Drupal is the #1 platform for web content management among global enterprises, governments, higher education institutions, and NGOs. Flexible and highly scalable, Drupal publishes a single web site or shares content in multiple languages across many devices. Technology and business leaders transform content management into powerful digital solutions with Drupal … backed by one of the world’s most innovative open source communities.
Drupal is open
Drupal was created in 2001 and released as open-source software under the GNU Public License. That means it’s completely free to download, use, and modify: there are no purchase, license, or maintenance fees.
Drupal is web development
Drupal’s default installation is a database-backed web platform with file maintenance tools and a high level of security. Many developers take advantage of Drupal’s extensible architecture to go far beyond the “core” package.
Drupal is social
Drupal is a multi-user system, letting site visitors log in (as “authenticated” users) or browse the site without doing so (as “anonymous” users). You control their access levels, and can also assign “roles” for multiple permission levels.
Drupal is content
Drupal both hosts and promotes your content. It can reach out beyond your site to give you a presence on Twitter, Facebook, and in other venues. Drupal’s built-in features also give search engines what they want, improving your web impact automatically.
Drupal is scalable
Drupal has proven itself repeatedly in the field on sites with over a million pages, and at over 20,000 requests per second. Its core installation handles over 99 percent of use cases, while free optimization tools and a large base of Drupal-trained administrators address the other one percent.
Drupal is design
Drupal site designs come from “themes”, whose visual presentation is described in standard CSS. Drupal.org hosts hundreds of free, fully customizable themes, including several “base themes” to help you launch your own designs.
Drupal is everywhere
Drupal runs millions of web sites, including many of the world’s biggest. It’s especially strong among major governments and industry-leading enterprises, but is easy enough to be the platform of choice for individuals and small companies.
Drupal is secure
Drupal is used by thousands of high profile web sites and is subject to rigorous security testing both by the Drupal community and by security experts around the world. Drupal's core code has been proven to prevent common security vulnerabilities such as those defined by the Open Web Application Security Project (OWASP).
silem.link: Also check out Yuriy Babenko’s explanation, which provides a different, yet even more useful perspective.