I notice that when I access a forum full of posts from several years, the forum page downloads in an intelligent way.   I can view the most recent posts, but if I keep hitting the PAGE-DOWN key, then the browser sends a message to the server to download more.  This is probably done somehow with Ajax.   It appears seamless to the user, there is just a small delay and a group of older posts are added to the bottom of the existing page visible in the browser.   The main clue that something is happening is the scoll bar changing on the right side of the browser.

I'd like to know how this works.   Can I emulate it in my own code?