The .NET framework class libraries are the core part of any .NET Framework programming language. It contains around 3400 classes classified logically into namespaces. Each class contains numerous methods and properties which you will use for your programming tasks. These libraries are available for any language under the Common Language Runtime environment.
A snapshot of the .NET Framework hierarchy is given in the figure below.
Furthermore, these libraries will be automatically installed at the time of installation of the .NET Software Development Kit. An interesting factor with regard to .NET is that if you are familiar with one language like C# it will be easy for you to migrate to other languages like Visual Basic without spending much time to learn new syntaxes and adapting to visual environments and conventions. However, there are some differences between C# and Visual Basic with regard to applicability of syntaxes. For instance, C# is a case sensitive language while Visual Basic is not. Moreover, Visual Studio contains single development environment for all .NET languages and hence the learning curve is further simplified. A complete list of .NET Framework class libraries is available at http://msdn.microsoft.com/en-us/library/d11h6832(vs.71).aspx