Http Handler and http Modules

ASP.NET handles all the HTTP requests coming from the user and generates the appropriate response for it. ASP.NET framework knows how to process different kind of requests based on extension, for example, It can handle request for.aspx, .ascx and .txt files, etc. When it receives any request, it checks the extension to see if it can handle that request and performs some predefined steps to serve that request.

Now as a developer, we might want to have some of our own functionality plugged in. We might want to handle some new kind of requests or perhaps we want to handle an existing request ourselves to have more control on the generated response, for example, we may want to decide how the request for .jpg or .gif files will be handled. Here, we will need anHTTPHandler to have our functionality in place.

There are also some scenarios where we are ok with the way ASP.NET is handling the requests but we want to perform some additional tasks on each request, i.e., we want to have our tasks execute along with the predefined steps ASP.NET is taking on each request. If we want to do this, we can have HTTPModule in place to achieve that.

So from the above discussion, it is clear that HTTPHandlers are used by ASP.NET to handle the specific requests based on extensions. HTTPModule, on the other hand, is used if we want to have our own functionality working along with the default ASP.NET functionality. There is one Handler for a specific request but there could be N number of modules for that.

For more detail visit 
Click here