Drupal - Create a hook function

Essentially there are 2 things needed to create your own hook(s).

• Create your own function that calls module_invoke_all('WHAT_YOU_WANT_YOUR_HOOK_TO_BE_CALLED');
• Create your new module and a function called YOURMODULE_WHAT_YOU_WANT_YOUR_HOOK_TO_BE_CALLED()

That is ALL it actually takes to implement your own hooks in a Drupal module. The great thing here is the flexibility that it provides for you to do anything, AND allow other developers to easily extend your work. The major consideration I can see here is naming your hook. You should obviously try to choose something unique to your module, that a core or another contributed module wont be using.

You don't create a hook, in the same way Drupal core doesn't create hook_nodeapi(), or hook_hook_info(). You write code that uses (invokes) the implementation of a hook done in other modules. The modules that want to interact/integrate with yours will implement that hook, and your module will use module_invoke(), module_invoke_all(), or drupal_alter() to invoke those hooks.

The correct phrase is not creating a hook, but defining a hook, which is essentially documenting that your module invokes a specific hook.