How is GMail Chat able to make AJAX requests without client interaction?

All HTTP responses require the client to initiate them, even those made using AJAX. But GMail's chat feature is able to receive messages from other users, even when I'm just sitting in my comfy computer chair watching but not interacting with the browser. How did they do it?

Live chat with PHP and jQuery. Where to store information? Mysql or file?

There are 1 on 1 live chat. Two solutions:

1) I store every message into database and with jQuery's help I check if there is a new message in database every second. Of course I use cache either. If there is, we give that message.

Developing Chat API like that of Stackoverflow

How to begin developing chat api, like the one stackoverflow uses? If it is open source, where can i find it, if not can anyone guide me how to build a similar chat api?