How to write custom Form API elements for Drupal 7?
Searching Google for "hook_element_info tutorial" brings up Create custom Form API elements with hook_element_info() in Drupal 7.

There is also the Examples for Developers project, which is included in the documentation at http://api.drupal.org. The Form API Example module implements hook_element_info() and contains a number of example elements in the form_example_elements.inc file.