Drupal’s hook system is essentially a mechanism for implementing custom features without needing to make any modifications to the Drupal core. There are specific places where the Drupal core can invoke custom functions defined in modules to enhance the functionality of core. These places are referred to as “hooks” and have a well-defined interface. Hooks make it possible, for example, for a module to define new URLs and pages within a site (hook_menu), to add content to pages (hook_block, hook_footer, etc.), set up custom database tables (hook_schema), and so on.