Rather than introduce the complexity of Object Oriented Programming, Drupal utilizes its "Hooks" system to achieve a similar result.  Hooks are specially named PHP functions having a defined set of parameters and a specified result type.

Using the hook naming convention, a hook function named foo_bar() indicates that a module named "foo" implemented a hook named "bar". When referring to a specific hook, the string "hook" is used as a placeholder for the module name. For example, for a module file called example.module implemeting hook_help, the hook would be named example_help().