Active Server Pages (ASP).NET is a server-side scripting environment that you can use to create and run dynamic, interactive Web server applications. ASP applications are easy to develop and modify.
An ASP.NET script begins to run when a browser requests an .aspx file from the Web server. The Web server then calls ASP.NET page, which reads through the requested file from top to bottom, executes any script commands, and sends a Web page to the browser.
Because your scripts run on the server rather than on the client, your Web server does all the work involved in generating the Web pages that you send to browsers. You need not worry whether a browser supports scripting, applets, or COM controls. The Web server does all the script processing, transmitting standard HTML to the browser. Server-side scripts cannot be readily copied because only the result of the script is returned to the browser. Users cannot view the script commands that created the page they are viewing
With the introduction of the support for Microsoft .NET Framework, you can now generate ASP.NET Web Forms which run on the Microsoft .NET Framework.