Drupal is a web development platform. It is sometimes called a "content management system", but that description seems insufficient. I would describe it as a "communication management system", because it profoundly can make development for communication between users more effective.  
Drupal offers features similar to other applications, but has advantages because of its original purpose. It was originally conceived and developed by Dries Buytaert, while he was a Masters degree student at the University of Antwerp in Belgium, to create a way for his friends to communicate.

Some good things about Drupal...

  • It's open source software (i.e. technically "free")
  • written in PHP
  • it is modular, with Core modules providing flexibility and power to create the foundation of a website, and available optional modules - some third-party, and some Drupal community developed - to extend site functionality

Drupal is used:

  • on 14% of the top 10,000 websites worldwide
  • for everything from government and political and corporate websites to personal blog sites
  • for functionality such as knowledge management, collaboration, retail e-commerce, and so much more

The community aspect of Drupal:

  • more than 1.39 million members
  • 124,000 actively contributing users
  • 48,300 free modules - to extend and customize Drupal functionality
  • 3,000 free themes
  • over 1,400 free "distributions" (pre-configured installations allowing quick, easy set-up of Drupal to providing specific "out-of-the-box" functionality)

Informational Links:  
- https://www.drupal.org/  
- https://en.wikipedia.org/wiki/Drupal