In Drupal's [Form API page](http://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7) mentioned both [hidden](http://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7#hidden) and [value](http://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7#val) will not display to user.

What is the difference between them?