An ASP.Net HTTP handler is the process that runs in response to a request that is made to an ASP.Net web application. The most common handler is an ASP.Net page handler that processes .aspx files. When user requests an .aspx file, the request is processed by the page handler.
ASP.Net page handler(*.aspx)
-> The default HTTP handler for all ASP.Net pages.
Web service handler(*.asmx)
-> The default HTTP handler for Web service pages created as .asmx files in ASP.Net
Generic Web handler(*.ashx)
-> The default HTTP handler for all web handlers that do not have a UI and that include the @WebHandler directive.
Trace handler (trace.axd)
-> A handler that displays current page trace information.