A hook is a type of function used in the Drupal CMS. It allows modules to interact with the core code of Drupal, or with other modules, without having to modify the original source code. Hooks are an integral part of the Drupal architecture, and are used to extend and modify the functionality of Drupal.
Hooks are written in PHP and are stored in the modules directory of the Drupal installation. When a module is enabled, Drupal will look for any hooks that are defined in the module and execute them. This allows modules to interact with the core code of Drupal, or with other modules, without having to modify the original source code.
Hooks are divided into two types: Actions and Filters. Actions are functions that are triggered when certain events occur, such as when a user logs in or when a node is saved. Filters are functions that modify data before it is displayed or stored. For example, a filter could be used to modify the output of a node before it is displayed on the page.
Hooks are an important part of the Drupal architecture, and are used to extend and modify the functionality of Drupal. By using hooks, developers can create powerful and flexible modules that interact with the core code of Drupal, or with other modules, without having to modify the original source code.