HttpHanlder is the low level Request and Response API to service incoming Http requests. All handlers implement the IHttpHandler interface. There is no need to use any extra namespace to use it as it contains in the System.Web namespace. Handlers are somewhat analogous to Internet Server Application Programming Interface (ISAPI) extensions.
Each incoming HTTP request received by ASP.NET is ultimately processed by a specific instance of a class that implements IHTTPHanlder. IHttpHanlderFactory provides the infrastructure that handles the actual resolution of URL requests to IHttpHanlder instances. In addition to the default IHttpHandlerFactory classes provided by ASP.NET, developers can optionally create and register factories to support rich request resolution.