What is the difference between a Shared Project and a Class Library in Visual Studio 2015?

I was looking at the new features for Visual Studio 2015 and Shared Project came up a lot but I don't understand how it is different to using a Class Library or a Portable Class Library. Can anyone explain?

Edit: Shared Project is a new feature in Visual Studio 2015 and is different to a Portable Class Library. I understand what a Portable Class Library is. What I'm trying to understand is how a Shared Project differs to a Class Library. See link below.

http://www.c-sharpcorner.com/UploadFile/7ca517/shared-project-an-impressive-features-of-visual-studio-201/

The difference between a shared project and a class library is that the latter is compiled and the unit of reuse is the assembly.

Whereas with the former, the unit of reuse is the source code, and the shared code is incorporated into each assembly that references the shared project.

This can be useful when you want to create separate assemblies that target specific platforms but still have code that should be shared.

See also here:

The shared project reference shows up under the References node in the Solution Explorer, but the code and assets in the shared project are treated as if they were files linked into the main project.

In previous versions of Visual Studio¹, you could share source code between projects by Add -> Existing Item and then choosing to Link. But this was kind of clunky and each separate source file had to be selected individually. With the move to supporting multiple disparate platforms (iOS, Android, etc), they decided to make it easier to share source between projects by adding the concept of Shared Projects.

¹ This question and my answer (up until now) suggest that Shared Projects was a new feature in Visual Studio 2015. In fact, they made their debut in Visual Studio 2013 Update 2

I found some more information from this blog.

• In a Class Library, when code is compiled, assemblies (dlls) are generated for each library. But with Shared Project it will not contain any header information so when you have a Shared Project reference it will be compiled as part of the parent application. There will not be separate dlls created.
• In class library you are only allowed to write C# code while shared project can have any thing like C# code files, XAML files or JavaScript files etc.

In-Short Differences are

1) PCL is not going to have Full Access to .NET Framework , where as SharedProject has.

2) #ifdef for platform specific code - you can not write in PCL (#ifdef option isn’t available to you in a PCL because it’s compiled separately, as its own DLL, so at compile time (when the #ifdef is evaluated) it doesn’t know what platform it will be part of. ) where as Shared project you can.

3) Platform specific code is achieved using Inversion Of Control in PCL , where as using #ifdef statements you can achieve the same in Shared Project.

An excellent article which illustrates differences between PCL vs Shared Project can be found at the following link

http://hotkrossbits.com/2015/05/03/xamarin-forms-pcl-vs-shared-project/