What is ASP .NET?

ASP .NET is a server-side scripting language developed by Microsoft Corporation. ASP.NET is a framework generally written in VB script or C#. The extension of asp.net file is .aspx which stand for Active Server Page and XML. ASP.NET pages are just scripts that tell IIS how to render a page.
It was first released in January 2002 with version 1.0 of the .NET Framework as the success of Microsoft's technology.

Wikipedia says:

ASP.NET is an open-source server-side web application framework designed for web development to produce dynamic web pages. It was developed by Microsoft to allow programmers to build dynamic web sites, web applications and web services.

ASP.NET is a web development platform, which provides a programming model, a comprehensive software infrastructure and various services required to build up robust web applications for PC, as well as mobile devices. It's built on the CLR (Common Language Runtime) which allow programmers to write ASP.NET code using any supported .NET language. The ASP.NET SOAP extension framework allows ASP.NET components to process SOAP messages.