An ASP.NET HTTP handler is a process that runs in response to a request made to an ASP.NET Web application. The most common handler is the ASP.NET page handler that processes .aspx files. When users request an .aspx file, the request is processed by the page through the page handler. Unlike ASP.NET Web Forms, that have ".aspx" file extension, ASP.NET handlers by default have ".ashx" file extension.