hook_block_info()

gistfile1.aw

<?php
/*
 * Implements hook_block_info()
 *
 * Adds a custom block
 */
function mymodule_block_info() {
  $blocks['ad'] = array(
    'info' => t('Advertisement'), 
    // DRUPAL_CACHE_PER_ROLE assumed
  );
  return $blocks;
}
?>