Formerly known as Project Neon, the Microsoft Fluent Design System is the latest iteration in the development of Microsoft's look-and-feel for Windows.
Fluent builds on the Metro design language introduced with Windows Phone. Metro was designed for touch devices in particular; with Fluent, Microsoft is aiming at devices ranging from those without any display at all, through phones, tablets, traditional PCs, to virtual and augmented reality systems. Fluent also marks a shift from a design primarily focused on consumption, to one that also incorporates content creation. This generally means that Fluent will have to scale to denser, more feature-rich interfaces than Metro ever did.
As well as broadening the scope of the new design approach, Microsoft is also trying to do a better job of getting designers and developers to understand it. The documentation for Fluent is already arguably more comprehensive than it ever was for Metro. It combines both design guidelines and developer references to show not just what to do but also how to do it.