More than one Drupal site on one machine

There are several possible configurations for running multiple Drupal servers on the same hardware. You can separate them by directories or by vhosts, they can share configurations or split them or, in some cases, have a mixture, but all of these methods have at their heart the ./sites/domain_or_host_name/settings.php configuration file and the search-sequence where the Drupal program will search first for a configuration named for the current page and then to the current host before settling for the default.