Each time a visitor visits a page with an AdSense tag, a piece of JavaScript writes an iframe tag, whose src attribute includes the URL of the page. Google's servers use a cache of the page for the URL or the keywords in the URL itself to determine a set of high-value keywords.

(Some of the details are described in the AdSense patent.) If keywords have been cached already, ads are served for those keywords based on the Adword bidding system.