HTTP handlers
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. One difference between HTTP handlers and
ISAPI extensions is that HTTP handlers can be called directly by using
their file name in the URL, similar to ISAPI extensions.
HTTP Modules
HTTP
modules are .NET components that implement the System.Web.IHttpModule
interface. These components plug themselves into the ASP.NET request
processing pipeline by registering themselves for certain events.
Whenever those events occur, ASP.NET invokes the interested HTTP modules
so that the modules can play with the request.