Описание тега ihttphandler

http://msdn.microsoft.com/en-us/library/system.web.ihttphandler(v=vs.100).aspx

All requests in the ASP.NET pipeline must eventually be handled by a class implementing the IHttpHandler interface. The handler is defined in either the system level web.config and can be overridden and website and subsite levels through the configuration inheritence chain.

The handler is usually mapped to one or more extensions (.aspx,.axd,.ascx, etc) may all be handled by Page Handler.

More details and explanation of how this interacts with the ASP.NET Request/Response pipeline may be found here.