The wp_ajax_ action is unfortunately poorly documented, there are nearly no references to it that are easy to follow / understand. The no_priv_ part just tells WordPress that the callback function will only be executed for users that aren't logged in into your wp-admin. The wpp_update part is the name of the action itself, you can call it whatever you want (eg. wp_ajax_nopriv_myaction).
Here is the document I used as reference and after a few days of struggling with it I finally managed to make my plugin work with WordPress' Ajax API. I might try and write a nicer tutorial about it someday, but fow the time being you can start there.
Anyways, back on topic.
Yes, the wp_ajax_nopriv_wpp_update is the hook that updates the database. Below it, you'll find another hook:
add_action('wp_head', array(&$this, 'wpp_print_ajax'));
These two hooks are needed for updating the database via WordPress Ajax API. However this action only works when the visitor isn't logged in, and for posts and pages views only.
Is this site of yours live now?