The Web Server - IIS

The first thing that happens to your request is IIS come in.
NB. I am aware that other web servers are beginning to support ASP.NET. But they will be totally ignored here.

The request comes into IIS and it does several things to it before it is sent to the ASP.NET process.

This diagram shows you that IIS does it's own processing of the request (to determine if it's valid etc.) and then looks for an ISAPI Filter for it. As IIS 5 and below only support ISAPI Filters as a means of handling the request, ASP.NET has it's own ISAPI Filter (called aspnet_isapi.dll). All this filter does is pass on the request to the ASP.NET Worker Process.

Of course, there is a lot of other stuff that goes on in there, but this is just the basics.