What is a Portable Class Library (new in .NET 4.5)?
The Portable Class Library project in Visual Studio 2012 RC enables you to write and build managed assemblies that work on multiple .NET Framework platforms. Using a Portable Class Library project, you choose the platforms (such as Windows Phone and .NET for Metro style apps) to target. The available types and members in your project are automatically restricted to the common types and members across these platforms. MSDN Article