An ASP.NET HTTP handler is the process (frequently referred to as the "endpoint") that runs in response to a request made to an ASP.NET Web application. 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. You can create your own HTTP handlers that render custom output to the browser.

An HTTP module is an assembly that is called on every request that is made to your application. HTTP modules are called as part of the ASP.NET request pipeline and have access to life-cycle events throughout the request. HTTP modules let you examine incoming and outgoing requests and take action based on the request.

Built-in HTTP Handlers in ASP.NET

An ASP.NET HTTP handler is the process (frequently referred to as the "endpoint") that runs in response to a request made to an ASP.NET Web application. 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 via the page handler.
 
The ASP.NET page handler is only one type of handler. ASP.NET comes with several other built-in handlers such as the Web service handler for .asmx files.
 
You can create custom HTTP handlers when you want special handling that you can identify using file name extensions in your application. For example, the following scenarios would be good uses of custom HTTP handlers:
 
RSS feeds   To create an RSS feed for a site, you can create a handler that emits RSS-formatted XML. You can then bind the .rss extension (for example) in your application to the custom handler. When users send a request to your site that ends in .rss, ASP.NET will call your handler to process the request.
 

Image server   If you want your Web application to serve images in a variety of sizes, you can write a custom handler to resize images and then send them back to the user as the handler's response.
 

HTTP handlers have access to the application context, including the requesting user's identity (if known), application state, and session information. When an HTTP handler is requested, ASP.NET calls the ProcessRequest method on the appropriate handler. The handler's ProcessRequest method creates a response, which is sent back to the requesting browser. As with any page request, the response goes through any HTTP modules that have subscribed to events that occur after the handler has run. For more information about the processing of Web server requests, see ASP.NET Application Life Cycle Overview.
 
An HTTP handler can be either synchronous or asynchronous. A synchronous handler does not return until it finishes processing the HTTP request for which it is called. An asynchronous handler runs a process independently of sending a response to the user. Asynchronous handlers are useful when you need to start an application process that might be lengthy and the user does not need to wait until it finishes before getting a response from the server.

HttpHandlers have a specific use and are not used in every situation..

What is a HttpHandler?

Well a HttpHandler is used in situations where you need to "handle" the "http" request sent to your webserver in a specific or different manor. More over a Http Handler (which is the basis of all .Net managed handlers) opens up the RAW http request (and response) allowing for complete customization in dealing with the request and providing a customized response.

When you use a HttpHandler ?

A Http Handler (as said above) simply recieves and responds to a http request. Typically we use our .net (aspx) pages to handle and respond to these request. However when it comes to specific situations, IE. request for downloading a file, displaying a chart image etc... It is usually better to remove all the "un needed" events, data and sub routines required to show a full page. Http Handlers can also provide a low level access to events and data from all http requests. This allows you to "intercept" http requests and process data \ header information before your application processes the request.

ASP.NET request processing is based on a pipeline model in which ASP.NET passes http requests to all the HTTP modules in the pipeline. Each module receives the http request and has full control over it. The module can play with the request in any way it sees fit. Once the request passes through all of the HTTP modules, it is eventually served by an HTTP handler. The HTTP handler performs some processing on it, and the result again passes through the HTTP modules in the pipeline.

During the processing of an http request, only one HTTP handler will be called, whereas more than one HTTP modules can be called.

HttpHandlers

HTTP handlers are the .NET components that implement the System.Web.IHttpHandler interface. Any class that implements the IHttpHandler interface can act as a target for the incoming HTTP requests. HTTP handlers are somewhat similar to ISAPI extensions. It is commonly used for url rewriting and handling particular extension like '.merit' through asp.net.
you can Register the handler by adding the following text in the web.config file:

<httpHandlers>
<add verb="*" path="*.merit" type="MyHandler.NewHandler,MyHandler"/>
</httpHandlers>
<http://www.15seconds.com/issue/020417.htm>