Drupal Core Hooks and API guide Links:
http://api.drupal.org/api/drupal/includes--module.inc/group/hooks/6
Hook_user:
http://api.drupal.org/api/drupal/developer--hooks--core.php/function/hook_user/6
Drupal FORM Api guide: This document provides a programmer's reference to the Drupal Forms API:
http://api.drupal.org/api/drupal/developer--topics--forms_api_reference.html/6