The HttpApplication is responsible for the request flow by firing events that signal your application that things are happening. This occurs as part of the HttpApplication.Init() method (look at System.Web.HttpApplication.InitInternal and HttpApplication.ResumeSteps() with Reflector) which sets up and starts a series of events in succession including the call to execute any handlers. The event handlers map to the events that are automatically set up in global.asax, and they also map any attached HTTPModules, which are essentially an externalized event sink for the events that HttpApplication publishes.

Both HttpModules and HttpHandlers are loaded dynamically via entries in Web.config and attached to the event chain. HttpModules are actual event handlers that hook specific HttpApplication events, while HttpHandlers are an end point that gets called to handle ‘application level request processing’.

Both Modules and Handlers are loaded and attached to the call chain as part of the HttpApplication.Init() method call.