HTTP Runtime Support
For advanced developers who want to use APIs as powerful as the Internet Server APIs (ISAPIs) that were available as part of Internet Information Services (IIS), ASP.NET offers the IHttpHandler and IHttpModule interfaces. Implementing the IHttpHandler interface gives you a means of interacting with the low-level request and response services of the IIS Web server and provides functionality much like ISAPI extensions but with a simpler programming model. Implementing the IHttpModule interface allows you to include custom events that participate in every request made to your application.
For those who are familiar with IIS, IHttpHandler objects are functionally similar to IIS ISAPI extensions, and IHttpModule objects are functionally similar to IIS ISAPI filters.