Cron is a time based UNIX scheduler, and Drupal has its own cron!
Drupal's cron is a bit different from UNIX cron, because it does not involve the system cron.
Since Drupal is a scripting application, it does not reside in memory permanently. When a page is requested via a Drupal site and the request is served, at the end of the request, Drupal checks to see when cron was last run. It is has been a while, it runs cron.