About Drupal multisite installations
Drupal can run multiple separate websites from a single codebase. The individual websites share code (the Drupal core, modules, and themes), but each website has its own database so they do not share content, configuration, settings, or displayed themes.
This configuration can be very helpful for global website management tasks, such as website upgrades, since you only have to upgrade one codebase. However, having multiple websites running from the same codebase can multiply problems such as security issues across your websites.