How does index.php work?

Hello Everyone,
This is a very basic question but maybe it needs some explanation from long time drupalers to make me understand.

How does page request that goes to index.php is handled for all pages in drupal?

Here http://drupal.org/node/270000 I see an explanation of how node works. But my question is how drupal knows from index.php a specific page request?

Suppose I navigate to drupal.org/project/themes, index.php is called right? If so, i understand that this code

require_once './includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);

loads all basic functions, enabled modules, logging etc. Rest I don't know.

I'm posting the short index.php code here for some ningas to explain the code.

require_once './includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);

$return = menu_execute_active_handler();

// Menu status constants are integers; page content is a string.
if (is_int($return)) {
  switch ($return) {
    case MENU_NOT_FOUND:
      drupal_not_found();
      break;
    case MENU_ACCESS_DENIED:
      drupal_access_denied();
      break;
    case MENU_SITE_OFFLINE:
      drupal_site_offline();
      break;
  }
}
elseif (isset($return)) {
  // Print any value (including an empty string) except NULL or undefined:
  print theme('page', $return);
}

drupal_page_footer();