Difference between Microsoft.IdentityModel and System.IdentityModel

I have just started working with Windows Identity Foundation and as I was developing using a code sample for Windows Identity Foundation, I received an error message regarding a project reference to System.IdentityModel.  When I briefly overlooked the System.IdentityModel namespace, I found many of the same classes as are available in the Microsoft.IdentityModel namespace.

I have been told to work with Windows Identity Foundation to develop Claims, however, I am unable to tell the difference between the 2 assemblies and set of namespaces. 

Could anyone please tell me what are the differences between Microsoft.IdentityModel and System.IdentityModel and how do I know when to use each respective namespace?

Thanks.