Overview of an HTTP Request :

The following list describes the request-processing flow :

1. When a client browser initiates an HTTP request for a resource on the Web server, HTTP.sys intercepts the request.
2. HTTP.sys contacts WAS to obtain information from the configuration store.
3. WAS requests configuration information from the configuration store, applicationHost.config.
4. The WWW Service receives configuration information, such as application pool and site configuration.
5. The WWW Service uses the configuration information to configure HTTP.sys.
6. WAS starts a worker process for the application pool to which the request was made.
7. The worker process processes the request and returns a response to HTTP.sys.
8. The client receives a response.

Detail of a HTTP request inside the Worker Process

ASP.NET Request Flow:

• IIS gets the request
• Looks up a script map extension and maps to aspnet_isapi.dll
• Code hits the worker process (aspnet_wp.exe in IIS5 or w3wp.exe in IIS6)
• .NET runtime is loaded
• IsapiRuntime.ProcessRequest() called by non-managed code
• IsapiWorkerRequest created once per request
• HttpRuntime.ProcessRequest() called with Worker Request
• HttpContext Object created by passing Worker Request as input
• HttpApplication.GetApplicationInstance() called with Context to retrieve instance from pool
• HttpApplication.Init() called to start pipeline event sequence and hook up modules and handlers
• HttpApplicaton.ProcessRequest called to start processing
• Pipeline events fire
• Handlers are called and ProcessRequest method are fired
• Control returns to pipeline and post request events fire

refer:

A low-level Look at the ASP.NET Architecture

IIS Architecture