1. The ASP.NET HTTP pipeline relies on IIS to receive the request.
2. When IIS receives an HTTP request, it examines the extension of the file.
3. If the file extension is associated with executable code, IIS invokes that code, in order to process the request.
4. Mappings from file extensions to pieces of executable code (.exe) are recorded in the IIS metabase.
5. When ASP.NET is installed, it adds entries to the metabase (associating various standard file extensions, including .aspx and .asmx), with a library called aspnet_isapi.dll.
6. When IIS receives an HTTP request for one of these files, it invokes the code in aspnet_isapi.dll, which in turn funnels the request into the HTTP pipeline.
7. Aspnet_isapi.dll uses a named pipe to forward the request from the IIS to an instance of the ASP.NET worker process, aspnet_wp.exe.
8. The aspnet_wp.exe worker process uses an instance of the HttpRuntime class to process the request.
Hope it is clear.....