ASP.NET is a request processing engine. It takes an incoming request and passes it through its internal pipeline to an end point, where you, as a developer, can attach code to process that request. This engine is actually completely separated from HTTP or the Web server. In fact, the HTTP runtime is a component that you can host in your own applications outside of IIS or any server side application altogether. For example, you can host the ASP.NET runtime in a Windows form.
A Low-level Look at ASP.NET Architecture
