ASP.NET is a Microsoft Technology and it is server side scripting language of Microsoft. It is a program that runs inside IIS and IIS comes as a free component with Windows servers.

ASP.NET also part of Window 2000 and XP Professional. When a browser requests an HTML file to the server, the server will return that file. When a browser requests an ASP.NET file, IIS passes the request to the ASP.NET engine on the server. And Finally, the ASP.NET file is returned to the browser as plain HTML An ASP.NET file is just same as an HTML file
An ASP.NET file has the file extension ‘.aspx’. The contain of An ASP.NET HTML, XML, and scripts.

One large feature of ASP.NET is that ASP.NET is web services.

ASP.NET includes a huge class library which was built by Microsoft. Because this class library is so huge, it encapsulates a huge number of common functions. For example, in case you desired to retrieve information from a database and display that information in a simple grid control through classic ASP, then you must write lots of code.

One of the most important goals of .NET was to permit developers to write an ASP.NET application using multiple programming languages. As long as each ASP.NET page contains programming language, you can mix & match different pages using different languages & they will work together seamlessly.