An HTTPHandler is a class that implements the IHTTPHandler interface,
which is mainly the one method:

void ProcessRequest(
   HttpContext context
)

An HTTPHandler gives you access to the request earlier in its
lifetime, which can be useful for simple tasks.

Scott Hanselman's article shows you a pretty quick example here:

http://www.hanselman.com/blog/CompositingTwoImagesIntoOneFromTheASPNETServerSide.aspx

One of the uses he found for HTTPHandlers was presenting an image of a
check to the user. No need for generating a control tree, events,
etc., so it made sense to handle it in a simpler way.

Hope this helps.

HTTPHandler can be called directly from the browser by using its file
name in the URL. In C++ you can create an ISAPI that could do the same
job but cannot be called from the URL, you have to configure it in
IIS.

I had pretty much same question a while back... Here is my answer (to
myself).

You have 2 ways to process browser request...
1. Write ASPX page
2. Write HTTP Handler.

HTTP handler is low level and usually not used to produce HTML.
ASPX page is a high level and mostly used to generate HTML page.

As a matter of fact ASPX page processing is implemented using HTTP handler.
People from Microsoft wrote an HTTP handler to find aspx page, create
WebControls, run it... send output to browser...

There is nothing you can not do with HTTP handler that you can not do with
ASPX page.. You just going to waste some runtime if you do not need services
provided by ASPX.

So here is use-case scenarios so you would get better understanding.

1. Create HTML page that has couple buttons and grid on it.. ----- Use ASPX
page.. It's easy to manipulate those Web controls ASPX page creates for
you...

2. Get an image from the database using imageId    ---- Use HTTP handler.
You do not need ASPX page... since you do not have webcontrols..
Just write an myimage.axd with will interpret parameter ImageId, get binary
data and send to browser using Response.BinaryWrite... call it in your HTML
pages <img src="myimage.axd?imageid=1">

Of course you could have written aspx page in case 2 but you would waste
some run time..
Or you could have written a HTTP handler in case 1 but it would remind old
asp style of writing application.

---------------------------------------------------------------------------
HTTPModule is totally different and allows you to tap into request before or
after page is processed...
Use-case scenarios...
1. Url rewrite
2. Security like checking that user have used SSL where needed..
3. IP banning...

George.