AJAX forms in Drupal are provided by the Form API.

• You write the form using the Drupal Form API.
• You specify what element activates the AJAX behavior.
• You specify what part of the form's HTML is to be replaced by the callback.
• You write a callback that selects and returns the piece of the form which gets replaced.
• Drupal manages everything so that when the button or control is used, the callback gets called and the replacement of HTML is done.

Although Javascript is involved (supplied by Drupal) you don't write any and you don't see any.