What is .NET Framework Architecture
The first level of the representation is the operating system; the .NET layer is located between the system and applications. The second level is the Common Language Runtime (CLR), which provides the part of the .NET Framework doing the most work. The next level is the Base Class Library (BCL), which provides all .NET objects that can be used both in your code and by Visual Studio when creating applications. The BCL also provides the infrastructure of several .NET technologies that we use in building applications, such as WPF, Windows Forms, ASP.NET, WCF, and so on. The last level is represented by applications that rely on the previous layers.
Differences With Previous Versions
If you upgrade to Visual Studio2010 from Visual Studio2008, the main difference that you notice is that .NET 4.0 is a standalone infrastructure. You may remember that .NET Framework 3.5 was instead an incremental framework that needed the prior installation of .NET 2.0 and .NET 3.0. For example, LINQ was part of .NET 3.5 whereas WPF was part of .NET 3.0 and Windows Forms was part of .NET 2.0 with .NET 4.0 this incremental structure disappears, and all the frameworks, BCL, and tools are part of the new version.