How to create generic handler and call it using javascript

Hi,
I am new to asp.net. I have a task to create a generic handler and call it.
Let me explain:
I added a file upload control to web page. Wrote a seperate class for functions and call that class on button click of upload.
Now, i want to convert this generic handlers concept. How could i do that?
When i was doing this using scenarion i explained it was easy to pass parameters which i stored in session, variables, hiddenfields.
Now, i dont know anything about generic handler. How could i do my task with handlers? If possible please post the code snippets.

Thanks,
Niraj