Drupal 8 is an open source content management system and powerful framework that helps deliver great websites to individuals and organizations, including non-profits, commercial, and government around the globe. This new release has been built on top of object-oriented PHP and includes more than a handful of improvements such as a better user experience, cleaner HTML5 markup, a new templating engine called Twig, multilingual capabilities, new configuration management, and effortless content authoring.
Twig
Twig Template naming conventions
Official Drupal documentation about Twig template naming convention.