A Universal Windows app was first introduced in Windows 8 as the Windows Runtime, which was built upon the Universal Application Platform. Now, in Windows 10, the name of the Universal Application Platform has been changed to Universal Windows Platform (UWP). You can build modern and fully immersive apps by targeting Windows 10 devices for Windows Store such as PC, tablet, phone, etc.
Universal Windows Platform (UWP) is a platform-homogeneous application architecture created by Microsoft and first introduced in Windows 10. The purpose of this software platform is to help develop Metro-style Universal apps that run on both Windows 10 and Windows 10 Mobile without the need to be re-written for each. It supports Windows application development using C# or XAML. As part of the core, the UWP now provides a common app platform available on every device that runs Windows 10. UWP apps automatically adjust the size of controls, fonts, and other UI elements so that they are legible on all devices.