Konstantin Kafer on the Form API States System
Konstantin Käfer (aka kkafer) talks about the Form API States System for Drupal 7. The states system allows you to create dependencies between form elements so that you can for example make a textbox disappear when a checkbox is checked. The Examples for Developers module has some more examples of the #states syntax on api.drupal.org. For more detailed information on how to use the states system, we cover it in Chapter 12 of the jQuery & JavaScript in Drupal Lullabot Learing Series video.