Use this project type to write and build portable .NET Framework assemblies, and then reference those assemblies from apps that target multiple platforms such as Windows and Windows Phone. [via <a href="http://msdn.microsoft.com/en-us/library/vstudio/gg597391(v=vs.100).aspx]">http://msdn.microsoft.com/en-us/library/vstudio/gg597391(v=vs.100).aspx]</a> Each platform has the concept of a class library that provides the implementation for each type of platform.