Magento has complex url rules, but to get you started quickly, most of the time you can assume its in the folowing format
router/controller/action
Router is defined by module, controller is defined by the controller class name (file name), action is the action method name in that controller. So if you open app/code/core/Mage folder, you will find a Customer folder, which is a module, an AccountController.php inside its controllers folder, and createAction in this controller file, which is what you are looking for.
