Drupal is a CMS (Content management system) developed in PHP since 2000 and originally created by Dries Buytaert.
Drupal 8 was published in 2015 and represented so much changes that it took around 2 years to be generally economically viable in production. This Drupal version now relies on PHP's best code as Symfony. It remains as easy to basically use as the previous version, but its development is now a lot more complex to master. To the contrary of Drupal 7 to Drupal 8 change, the change to Drupal 9 should be relatively easy to do.
Drupal 7 was published in 2011 and is still common in 2020, at least on already developed websites. It's more simple to develop than with Drupal 8 but this version is really in its end of life and clearly lacks the main technological evolutions of those last years. For new websites it must therefore be avoided.
Drupal 9 is in development.
Drupal allows to manage an impressive number and variety of content.
Drupal relies on extremely powerful technologies such as Symfony, Composer, ... and proposes ambitious APIs as Entity, Views, ...
Drupal community is one of the most important opensource communities in the world, with more than 1 million developers, site builders, managers, ...