If you are going down the river, the hook is probably not very popular, but in Drupal´s world it is the essential basis of everything.
The core browse progressively through the list of hooks and considers every permitted module to “hook up” if this module implements the hook. You can imagine the particular hook as an event, for example, deleting of the user. Hook shows how the Drupal´s core servers all modules. If there is the order (display the page, store the element, delete the file..), the core browse through the list of hooks and the module has a chance to hook up. The hook is implemented by this module. You can imagine the particular hook as an event. The Drupal´s core has a number of such hooks (the list here) but you have the possibility to implement your own hook to work with events important for your application.
