Drupal 8 is a partial modernisation of the legacy Drupal CMS with a few parts of Symfony under the hood. Usability improvements were not the focus and to the end user Drupal 8 is very much like Drupal 7.
Drupal is a Content Management system that uses some Symfony Components. With Drupal 8 you're working with a custom structure enforced by Drupal, but you see a lot of familiar concepts and syntax from other projects utilising Symfony. After years of development, Drupal 8 was released late in 2015.
Curiously, performance has received a downgrade in the modernisation process of the back end technology. This partly undermines the credibility Drupal 8 as a CaaS backend for content requiring authentication. For site creation the improved caching alleviates this bottleneck.
The previous versions of Drupal have been subject to some serious vulnerabilities. Including the link between Drupal and the Panama Papers leaks.