The Universal Windows Platform (UWP) introduces a common API that you can use to deliver unique and personal app experiences on all devices that run Windows 10. In this section you’ll find comprehensive API reference info for developing Universal Windows apps for Windows 10, or Windows Runtime apps for Windows 8.x. There’s also reference for the main programming languages from which you can call Windows APIs..
If you're new to Universal Windows apps, see What's a Universal Windows app? for a brief intro.