What are hooks in Drupal ?

Allow modules to interact with the Drupal core.

Drupal’s module system is based on the concept of “hooks”. A hook is a PHP 

function that is named foo_bar(), where “foo” is the name of the module 

(whose filename is thus foo.module) and “bar” is the name of the hook. 

Each hook has a defined set of parameters and a specified result type.
 

To extend Drupal, a module need simply implement a hook. When Drupal wishes 

to allow intervention from modules, it determines which modules implement a hook and calls that hook in all 

enabled modules that implement it.