In this blog I’ll discuss about Client Profile which is nothing but a subset of the associated framework and optimized for client applications e.g. .NET Framework 4 Client profile is a subset of .NET Framework 4.
You can view or change the target framework for a project in Visual Studio 2010 as displayed below
Client Profile improves the deployment experience by having smaller download sizes and quicker install times as it has a smaller redistribution package that installs the minimum set of client assemblies on the user's computer, without requiring the full version of the .NET Framework to be present.
If an assembly is referred which is not in Client Profile then the full version of .NET Framework is required. For more details please refer this msdn article .NET Framework Client Profile