A Universal Windows app is a Windows experience that is built upon the Universal Windows Platform (UWP), which was first introduced in Windows 8 as the Windows Runtime. At the core of Universal Windows apps is the idea that users want their experiences to be mobile across ALL their devices, and they want to use whatever device is most convenient or productive for the task at hand.
Windows 8.1 and Windows Phone 8.1 apps target an operating system (OS): either Windows, or Windows Phone. With Windows 10 you no longer target an operating system but you instead target your app to one or more device families. A device family identifies the APIs, system characteristics, and behaviors that you can expect across devices within the device family. It also determines the set of devices on which your app can be installed from the Store.
With Windows 10 it has become easier to develop apps for the UWP with just one API set, one app package, and one store to reach all Windows 10 devices – PC, tablet, phone and more. It’s easier to support a number of screen sizes, and also a variety of interaction models, whether it be touch, mouse & keyboard, a game controller, or a pen.
Microsoft Visual Studio 2015 provides a Universal Windows app template for each language that lets you create a single project for all devices. When your work is finished, you can produce an app package and submit it to the Windows Store from within Visual Studio to get your app out to customers on any Windows 10 device.
Windows 10 and new developer tools provide the tools, features, and experiences powered by the new Universal Windows Platform. After installing the tools and SDK on Windows 10, you’re ready to either create a new Universal Windows app or explore how you can use your existing app code on Windows.
Source: Microsoft Developer Network