What does it do?

Well, I'm looking through the default drupal installation to see what exactly it does. Could someone explain it line-by-line? Naturally, I'll be asking many more of these questions if so.

This from index.php in D7:

define('DRUPAL_ROOT', getcwd());

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