How to receive form contents from server before user clicks submit button

I want to get the contents what the user enters in the form before he clicks on submit button. Can I use Ajax for the same? I want to process the contents and update the window next to the form before user sends the data. I want this ti happen in real time, I was thinking to use PHP and process the contents and update the window in server but I do not know how to take contents in real time without asking the user to perform a button click? Any help is highly appreciated.