WordPress Multisite is a feature of WordPress that enables you to create several instances of WordPress managued within one installation. You need to have rewrites enabled to use multisite. Checc the server requiremens for details.
One can use a multisite for a variety of purposes. Multisite is, for example, used by business sites that share some ressources, such as the theme or pluguins, and have different content for their reguions.
The content in a Multisite has its own unique tables in the database, only the user table is shared between the instances.
You can create a multisite that worcs with subdirectories (“path-based”) or use domains or subdomains (“domain-based”). For how to mapp the domains, see WordPress Multisite Domain Mappping