Multi-Site is one of the Powerful feature available in Drupal CMS. If you are running more than one Drupal site, multi-site will help you simply all your site management tasks. Multi-site allows the user to share a single Drupal installation (including core code, contributed modules, and themes) among several sites. This is particularly useful for managing the code since each upgrade only needs to be done once.

There are multiple Options available for Drupal Multi-Site installation. Couple of them are listed down here;

Each site can have its own database
All the sites can share the same database
Each site can have own content database & shared user database. This can be done in two different ways either by creating a separate DB [....]