Drupal is a Free and Open Source content management framework written in PHP and distributed under the GNU General Public License.
Drupal is content management software. It’s used to make many of the websites and applications you use every day. Drupal has great standard features, like easy content authoring, reliable performance, and excellent security. But what sets it apart is its flexibility; modularity is one of its core principles. Its tools help you build the versatile, structured content that dynamic web experiences need.
The Drupal project is open source software written in PHP. Anyone can download, use, work on, and share it with others. It’s built on principles like collaboration, globalism, and innovation. It’s distributed under the terms of the GNU General Public License (GPL). There are no licensing fees, ever. Drupal will always be free.
GovCMS is also built on Drupal, with emphasis placed on the strict security and compliance needs, generally required by Government clients.
Find more info here: https://www.drupal.org/