HttpHandlers are the earliest possible point where we have
access to the requests made to the web server (IIS). When a request is made to
the web server for an ASP.NET resource (.aspx, .asmx, etc.), the worker process
of the ASP.NET creates the right HttpHandler for the request which responds to
the request. The default handlers for each request type that ASP.NET handles
are set in the machine.config file. For example, all the requests made to ASP.NET
(.aspx) pages are handled by System.Web.UI.PageHandlerFactory. So whenever an ASP.NET
is requested at the web server, the PageHandlerFactory will fulfill the
request.