Read original post at Matteo Pagani's Blog
One of the most important features that was added in Windows Phone 8.1 is the native Windows Runtime support: this new approach opened the doors to Universal Windows app, a new way to write applications for Windows and Windows Phone so that you can share most of the code. One of the biggest limitations in the Microsoft world, in the past, was that, despite the fact that Windows 8 and Windows Phone 8 have always shared many similarities (the application lifecycle, the isolated storage, the navigation approach, etc.), you ended to write two different applications, due to the many differences within the APIs implementation.