Microsoft has its own design language which is called Fluent Design System. There is also Fluent UI which is a component library. To my understanding, it's based on Fluent Design System (please correct me if I am wrong). Moreover, Microsoft has one more component library called UI Fabric. It is a part of the Fluent Design System as well.
If they both are based on the Fluent Design System, what is a difference between them? Will the Fluent UI library and the UI Fabric library be merged in the future? When developing a web app/site, which library to use?