The Portable Class Library project enables you to write and build managed assemblies that work on more than one .NET Framework platform. You can create classes that contain code you wish to share across many projects, such as shared business logic, and then reference those classes from different types of projects.
Using the Portable Class Library project, you can build portable assemblies that work without modification on the .NET Framework, Silverlight, Windows Phone 7, or Xbox 360 platforms. Without the Portable Class Library project, you must target a single platform and then manually rework the class library for other platforms. The Portable Class Library project supports a subset of assemblies from these platforms, and provides a Visual Studio template that makes it possible to build assemblies that run without modification on these platforms. [Source = MSDN]
The portable libraries project ships as part of Visual Studio 11 Developer Preview. You can build portable .NET class libraries by simply creating a Portable Class Library project (template provided for you) and choosing the platform targets. The IDE will then create the appropriate reference assemblies for you and you can then program as you normally would.
Here, we meet Portable Libraries project developer David Kean and program manager Mircea Trofin to learn more. Whiteboarding included.