• An API is a software-to-software interface, not a user interface.
  
• An API resembles Software as a Service (SaaS).
  
• Josh Walker says , an analyst at Forrester Research Inc. in Cambridge, Mass., 
  

"Building an application with no APIs is basically like building a house with no doors. The API for all computing purposes is how you open the blinds and the doors and exchange information."

• To extend Drupal, a module need simply implement a hook.
• 
  
• e.g. hook_help()

/**
 * Implements hook_help().
 */
function demo_help($path, $arg) {
  if ($path == 'admin/help#demo') {
    return t('A demonstration module.');
  }
}