Just as ASPX files can be compiled on the fly (just-in-time), so can handlers. Generic handlers have an extension of ASHX. They're equivalent to custom handlers written in C Sharp or Visual Basic.NET in that they contain classes that fully implement IHttpHandler. They're convenient in the same way ASPX files are convenient. You simply surf to them and they're compiled automatically.
It works in just the same way as the handlers implemented in .cs and .VB file.
ASP.NET HTTPHandlers are responsible for intercepting requests made to your ASP.NET web application server. They run as processes in response to a request made to the ASP.NET Site. The most common handler is an 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.
<add verb="POST,GET" path="ajax/*.ashx"
type="Ajax.PageHandlerFactory, Ajax" />