The Universal Windows Platform (UWP) was introduced with Windows 10. The UWP provides a common app platform (that is: core APIs) on every device that runs Windows 10.
If these core APIs are used, an app will run on any Windows 10 device, such as
Desktop PC
Xbox
Mixed-reality headset
tablets
phones
…
Windows Runtime
UWP apps use the Windows Runtime.
The Windows Runime is written in C++ and provided by Windows.
Misc
UWP applications automatically—and dynamically—scale for each display that they're running on.
All UWP Apps are also Window Apps, but not all Window Apps are UWP Apps. A Window App is not the same as a Win32 App.
TODO
Apparently, Windows needs to be put into developer mode to debug UWP apps.
UWP is one of the four primary implementations of .NET