Drupal Form API Tutorial - buildamodule.com
The Drupal Forms API is one of the most powerful tools in Drupal, and it can save you a ton of time if you use it right. In this tutorial, we're going to cover techniques for using the Forms API to generate forms, validate them upon submission and process them once they've passed. If you come from a code-from-scratch background, you know how difficult securing forms from abuse can be. Even though the Forms API looks a bit odd at first, once you grasp the concepts you'll see how much of the heavy lifting is done by Drupal and you'll never look back.