Drupal’s hook system allows for modules to interact and alter the workings of other modules or even Drupal core itself. It is a very simple system that allows modules to even create their own very easily. In common practice, there are two types of hooks that you would want to create - alter hooks and intercepting hooks. Alter hooks provide a com…

Drupal Design Patterns.php

// Example #1 (simple invoking)
<?php
  // will call all modules implementing hook_hook_name
  module_invoke_all('hook_name');
?>

//Example #2 (aggregate results)

<?php
  $result = array();
  foreach (module_implements('hook_name') as $module) {
    // will call all modules implementing hook_hook_name and 
    // push the results onto the $result array
    $result[] = module_invoke($module, 'hook_name');
  }
?>

//Example #3 (altering data using drupal_alter)
<?php
  $data = array(
    'key1' => 'value1',
    'key2' => 'value2',
  );
  // will call all modules implementing hook_my_data_alter
  drupal_alter('my_data', $data);
?>

//Example #4 (passing by reference cannot use module_invoke)
<?php
  // @see user_module_invoke()
  foreach (module_implements('hook_name') as $module) {
    $function = $module . '_hook_name';
    // will call all modules implementing hook_hook_name
    // and can pass each argument as reference determined
    // by the function declaration
    $function($arg1, $arg2);
  }
?>