When you click on a fruit it will grab the element, extract its contents and send it to a php file with a POST argument that goes like "user_pref_fruit=appeles"
The server will give some answer that will look like this:
<?xml version="1.0" encoding="ISO-8859-1" ?>
<answer>Oh you like bananas!</answer>
The source code for the JS part is in this page (do "View Source") and the PHP part is ajax1.phps