When building forms, you will often want to only provide certain options if other options are chosen by a user. For example, there's no need to show the 'open link in new window' checkbox, if the 'make this into a link' checkbox hasn't been ticked. These kinds of dynamic forms haven't been easily available for Drupal... until now, with the
#states for form elements in Drupal 7.
Take a peek at this example of dynamic forms in Drupal 7.
To make one form element be dependent on another element, you may have delved into AJAX, or tried using CTools...