I see websites like pof.com do this and I'm curious how it's done.

Basically I'm referring to how a website can be constructed so that when people submit information to the server through forms (or w/e other ways it can be done) if they click the back button they do not have to re-submit form data. I see it quite often on websites so I know it's possible and I don't think it's an Ajax thing...

Thanks in advance.