HTTP Handlers and HTTP Modules Overview

To learn about HTTP Handlers. Read on:

http://msdn.microsoft.com/en-us/library/bb398986.aspx

How to: Register HTTP Handlers

http://msdn.microsoft.com/en-us/library/46c5ddfy.aspx

Creating an ASHX handler in ASP.NET:

http://www.aspcode.net/Creating-an-ASHX-handler-in-ASPNET.aspx