The Drupal Forms API provides developers with complete control over how forms work within Drupal. By using the Forms API even complex tasks such as working with AJAX are made simple. The Forms API varies a bit between major versions of Drupal but the principles remain the same, take a look at what we've blogged about Forms over the years below...
There's an excellent article over on gtrlabs: Drupal 5: How to process multiple instances of the same form on the same page that describes in detail how to have multiple copies of the same form on the same page. There are some subtle differences to use this technique in Drupal 6, which I'll explain below. The idea of this technique is simple, drupal forms are identified by their 'form_id' and these ids must be unique...