1. Drupal core, modules, and themes
From an architectural standpoint, we can break up Drupal into three pieces--its core,
modules, and themes.
When we discuss Drupal 8 core, we can interpret it in two ways. A more restrictive
interpretation sees it as a functionality covered in all the code it ships with, without
modules and themes. The more widespread interpretation sees it as the total code base it
ships with (out of the box).