Everything else is finally just details -- significant details, but variations on this theme. index.php is the control, the menu method determins what the "current page" is, and plenty of occasions get fired in the process of construction that page. Plugin modules can hook into those events and change the work flow/furnish additional info/etc. That is also a part of the reason a lot of Drupal resources concentrate on making modules. Without modules, Drupal does maybe not actually DO anything other than say, 'Some one asked to get a page! Does this exist? No? OK, I'll serve up a 404.' Corro - lolz