Please refer to the following URL to know the differences between HTTP Modules and HTTP Handlers,http://onlydifferencefaqs.blogspot.in/2012/08/difference-between-httphandler-and.html

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.