http packets in dynamic web pages

I want to know in web pages like Yahoo wh开发者_如何学JAVAich every for example 5 min news changes, do we send http packet to server? If I want to implement this feature with AJAX do I need to send http packet again? Can some one explain the dynamic structure(like this updating news)for me.

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You can easily find out what's going on on the page: For example use Firefox and download the Firebug addon. Then open firebug after installation and open the Network tab. You may have to enable it. Reload your page in question, you'll see evere request made. And of course you will see timed requests done later to update the page. The details are pretty thorough.

This is how the firebug network tab looked like after posting the first paragraph of this answer: