What is a WordPress Multisite?
A network of websites that uses the same WordPress installation - core files, database, themes and plugins. Run from a central dashboard, but with the ability to create and manage websites individually, the overall management and maintenance of the system is streamlined and very efficient.
A multisite is ideal if the websites within it are for the same organisation or their representatives. They are especially useful where the cost of setting up and managing separate but similar website installations is prohibitive.
If your websites are for completely different organisations, have very different purposes, or have specific technical or legal requirements, you should consider separate installations for each website.