Fluent Design System, introduced at Microsoft's Build conference, is the company's new "design system" for Windows, akin to Google's Material Design for Android and its apps for non-Google platforms.
Eventually Fluent Design will be the basis for Microsoft's iOS and Android apps too, but some of the first elements are coming to the Fall Creators Update due out around September.
The design overhaul will take place gradually as Microsoft updates its apps and as more Windows developers adopt the various tools Microsoft is releasing to bring a focus on light, depth, motion, material, and scale in apps.
The tools include Acrylic Material, which offers apps an opaque layered effect to add depth, Connected animation for creating transitioning scenes, while Parallax creates a 3D effect, and Reveal offers a lighting effect that draws attention to interactive elements of an app.
Minor as these changes may seem, eventually they will affect all developers building apps for Windows. The softened layer will be present in any XAML-based UI and in apps that use Acrylic Material and target build 16251 and later.