Glossary - H
A hack is a bad practice. This is the action of modifying base files or contributed modules instead of using the Drupal hook system. Drupal Hacking makes it difficult to apply updates. A good way to check if your site has been hacked is to check using the https://drupal.org/project/hacked module.
A hook is a function used during Drupal module design that allows a programmer to modify an existing Drupal feature.