How did they make the Facebook Chat program not reload on changing page?

[xres74]

I am a website developer, and I am currently working on an on-site chat program with functionality similar to the current Facebook Chat program (the one that's fix-positioned on the bottom of the screen) on the Facebook website. One thing I've noticed is that their chat program does not get refreshed when you click on the various links or change pages on the site. This is very important, because you wouldn't want to lose your conversation with someone just because you clicked on something. But how is that done? How can you create a <div> or <iframe> that's not only shared on all pages, but also do not get reloaded when pages change? I don't think it's just ajax.