ASP .NET 4.0 :

ASP.NET is a Web platform that provides all the services that you require to build enterprise-class server-based Web applications. ASP.NET is built on the .NET Framework, so all .NET Framework features are available to ASP.NET applications.

Your applications can be written in any language that is compatible with the common language runtime (CLR), including Visual Basic and C#.
To create ASP.NET Web applications, you can use Visual Studio. The tools and options in Visual Studio that are designed for creating Web applications are referred to collectively as Visual Web Developer. In addition, a free standalone product—Visual Web Developer Express—is available that includes the core set of Web-design features from Visual Studio.
The following topics provide more information about ASP.NET and Visual Web Developer.