Extending The Core
Drupal core is modular, defining a system of hooks and callbacks, which are accessed internally via an API. This design allows third-party contributed (often abbreviated to "contrib") modules and themes to extend or override Drupal's default behaviors without changing Drupal core's code.
Drupal isolates core files from contributed modules and themes. This increases flexibility and security and allows administrators to cleanly upgrade to new releases without overwriting their site's customizations. The Drupal community has the saying "Never hack core", a strong recommendation that people do not change core files.
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