Windows 10 Universal Windows Platform (UWP) ..

Universal Windows Platform (UWP) which provides a guaranteed core API layer across devices. You can create a single app package that can be installed onto a wide range of devices. A single store makes it easy to publish apps across all device types. Because your UWP app runs on a wide variety of devices with different form factors and input modalities, you want it to be tailored to each device and be able to unlock the unique capabilities of each device. Devices add their own unique APIs to the guaranteed API layer. You can write code to access those unique APIs conditionally so that your app lights up features specific to one type of device while presenting a different experience on other devices. Adaptive UI controls and new layout panels help you to tailor your UI across a broad range of screen resolutions.

The Windows Runtime (WinRT) is the technology that lets you build Universal Windows Platform (UWP) apps. 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. Universal Windows apps are most often distributed via the Windows Store (but can also be side-loaded), and are most often packaged and distributed using the .APPX packaging format. Windows on ARM will only support WinRT, and not Win32.Those application which are metro application they has to go through WinRT & Classic Applications has to go through WIN32 Api's.

Visual Studio provide a Universal Windows app template that lets you create a Windows Store app (for PCs, tablets, and laptops) and a Windows Phone Store app in the same project. When your work is finished, you can produce app packages for the Windows Store and Windows Phone Store with a single action to get your app out to customers on any Windows device. You can create Universal Windows apps using the programming languages you're most familiar with, like JavaScript, C#, Visual Basic, or C++. You can even write components in one language and use them in an app that's written in another language. Universal Windows apps can use the Windows Runtime, a native API built into the operating system. This API is implemented in C++ and supported in JavaScript, C#, Visual Basic, and C++ in a way that feels natural for each language.

The following are the conditional compilation constants that you can use to write platform-specific code:

C#	WINDOWS_APP	WINDOWS_PHONE_APP
C++	WINAPI_FAMILY_PC_APP	WINAPI_FAMILY_PHONE_APP

ref:

Guide to Universal Windows Platform (UWP) apps - https://msdn.microsoft.com/en-us/library/Dn894631.aspx

Build a Windows 10 universal app - https://msdn.microsoft.com/library/windows/apps/xaml/dn609832.aspx#target_win10

Best practices in developing (universal) apps for Windows Runtime - https://github.com/futurice/windows-app-development-best-practices

Port your app to the Universal Windows Platform (UWP) -

1. https://msdn.microsoft.com/en-us/library/windows/apps/Dn751495.aspx
2. https://msdn.microsoft.com/en-us/library/dn986839(v=vs.140).aspx

How to: Use Existing C++ Code in a Universal Windows Platform App - https://msdn.microsoft.com/en-us/library/mt186162.aspx

Porting Android Apps to Windows 10 - http://venturebeat.com/2015/05/01/everything-you-need-to-know-about-porting-android-and-ios-apps-to-windows-10/

VC++ Team Blog - 

1. http://blogs.msdn.com/b/vcblog/
2. http://blogs.msdn.com/b/vcblog/archive/2015/03/03/introducing-the-universal-crt.aspx
3. http://code.tutsplus.com/tutorials/creating-your-first-universal-windows-app--cms-23122
4. http://blogs.msdn.com/b/vcblog/archive/2015/07/20/visual-studio-2015-rtm-now-available.aspx

Universal Windows Platform(UWP) sample code - 

1. https://msdn.microsoft.com/en-us/library/windows/apps/Dn751495.aspx
2. https://github.com/Microsoft/Windows-universal-samples

Microsoft Virtual Academy resources -

1. http://www.microsoftvirtualacademy.com/training-courses/a-developers-guide-to-windows-10
2. http://www.microsoftvirtualacademy.com/training-courses/developing-universal-windows-apps-with-c-and-xaml
3. http://www.microsoftvirtualacademy.com/training-courses/developing-universal-windows-apps-with-html-and-javascript-jump-start
4. http://www.microsoftvirtualacademy.com/training-courses/developing-games-with-marmalade-and-c-for-windows-and-windows-phone

UWP Channel9 Videos -

1. https://channel9.msdn.com/Series/A-Developers-Guide-to-Windows-10
2. https://channel9.msdn.com/Events/TechEd/Australia/2014/WPD305
3. https://channel9.msdn.com/Series/Creating-a-Universal-App-in-VB/UWP-port-from-81-universal-app-to-Win10