Universal Apps on Windows now run on a common software core, or kernel with a common runtime. The only remaining difference between the platforms has mostly to do with how they handle user-interface issues across a variety of devices, input methods (touchscreens vs. mouse and keyboard), hardware (CPU, memory, graphics processors, accelerometers and other sensors) and screen sizes.
The MSDN Code Gallery also has a large collection of feature samples that use the new universal Windows app template identified with the ‘universal app’ tag.
Project templates for universal apps are installed via Visual Studio 2013 with Update 2 or later. There are several Universal app samples available by Microsoft listed here.