AHAH in Drupal is a feature provided by the Forms API.
- You write the form using the Drupal Forms API.
- You specify what button activates the AHAH behavior.
- You specify what part of the form's HTML is to be replaced by the callback
- You write a callback that generates the replacement HTML
- Drupal manages everything so that when the button or control is used, the callback gets called and the replacement of HTML is done