hooks:- hooks are api code in drupal which allows interaction between module and Drupal core.
Basically A hook is a PHP function look like -
general is the name of the module (file name for whom is general.module).
test is the name of the hook.
Each hook has a set of parameters and dependent call back functions to trigger there functionality for a particular event.
A simple example to understand hook is you use form with a submit button, and submit button event you can have particular functionality.i.e. indicates hooks in drupal to allow these kind of functionality to do customly in the site as per requirement and called as hooks in drupal.