A Universal Windows Platform (UWP) app can run on any Windows-based device, from your phone to your tablet or PC. You can even create UWP apps that run on compact devices, such as wearables or household appliances.
When you design a UWP, you create a user interface that suits a variety of devices with different display sizes. To make that easier, we give you a set of universal controls that automatically work well on all devices. The platform does the work behind the scenes to ensure that text and visuals scale between devices and are always legible.
You can use the same code and design for all devices, and you can also tailor the user interface for specific screen sizes. For example, you can design an interface that works great for tablets and PCs and create a customized experience for mobile devices, while still reusing most of your code.
Looking for Windows 8.1 design guidance? You can get it as a PDF: Download the Windows 8.1 guidelines.