ASP.NET- HTTPModule and HTTPHandler

HTTP handlers: Extension based processing

HTTP Module : Event based processing

Typical uses for custom HTTP handlers include the following:

RSS feeds To create an RSS feed for a Web site, you can create a handler that emits RSS-formatted XML. You can then bind a file name extension such as .rss to the custom handler. When users send a request to your site that ends in .rss, ASP.NET calls your handler to process the request.

Image server If you want a Web application to serve images in a variety of sizes

Typical uses for HTTP modules include the following:

Security

Statistics and logging

Custom headers or footers Because you can modify the outgoing response, you can insert content such as custom header information into every page or XML Web service response.

(http://msdn.microsoft.com/en-us/library/ms227673.aspx)