Too Long; Didn't Read
WordPress Multisite allows several websites to be run from a single WordPress installation and database. The multisite system, formerly known as WordPress Multi-User (WPMU), isn't a new concept. Because you're only utilizing a single installation, you can maintain core, themes, and plugins up to date in a single dashboard. Because all your sites will be affected if the database goes down, there is an increased security risk. It is considerably simpler to back up and manage individual sites, when properly maintained, with fewer resources than several.