Difference between ASP.NET HttpHandler and HttpModule

For example when an .aspx page is requested it is passed to ASP.Net page handler. Then Application domain is created and after that different ASP.Net objects like Httpcontext, HttpRequest, HttpResponse are created. Then instance of HttpApplication is created and also instance of any configured modules. One can register different events of HttpApplication class like BeginRequest, AuthenticateRequest, AuthorizeRequest, ProcessRequest etc.

HTTP Handler

HTTP Handler is the process which runs in response to a HTTP request. So whenever user requests a file it is processed by the handler based on the extension. So, custom http handlers are created when you need to special handling based on the file name extension. Let's consider an example to create RSS for a site. So, create a handler that generates RSS-formatted XML. Now bind the .rss extension to the custom handler.

HTTP Modules

HTTP Modules are plugged into the life cycle of a request. So when a request is processed it is passed through all the modules in the pipeline of the request. So generally http modules are used for:

Security: For authenticating a request before the request is handled.

Statistics and Logging: Since modules are called for every request they can be used for gathering statistics and for logging information.

Custom header:  Since response can be modified, one can add custom header information to the response.

1, HTTPModule: 

    It's just like a filter. The Modules are called before and after the handler executes.
    For example: BeginRequest, AuthenticationRequest event, EndRequest event etc. You may intercept , participate and modify each request.
    Modules implement the IHttpMudule interface located in the System.Web.System.

2, HTTPHandler:

    You may think it as a  program.(or handler, or module), it execute some code when the user send some request. An .aspx page can be thought as a HTTPHandler too, which implements more functions.