With this release of the Windows developer platform of universal Windows apps, Microsoft sets out to accomplish three major goals: 1) Reach customers across phones, tablets, and PCs; 2) Deliver innovation that supports developer investments; 3) Make cross-platform technology easier and more capable.
Starting Windows Phone 8.1, it is now possible to use the Windows Runtime to build apps that can target each form factor in the family of Windows devices. Using the universal Windows app project templates, you can now create one app where your business logic can be surfaced up through a user experience relevant to the device. This unified approach to app development means that your app has the potential to reach not just Windows phones and tablets, but Windows laptops and workstations as well.
Creating a new Visual Studio solution using the shared project template will produce a Visual Studio solution configuration that includes a shared project space where you can define the code to share between an app targeted for tablet & PC and another targeted for phone. It will also establish separate projects for elements of the app that are unique to either. A number of improvements is included in Visual Studio 2013 Update 2 to support universal Windows app development for all Windows devices.
If you already have a Windows Store app, you can easily add a Windows Phone Store app to the same solution. Similarly, if you started by creating a Windows Phone app, you can easily add a Windows Store app.