AdSense Link units display a list of topics relevant to the content of your page. When a user clicks on a topic, a page of related ads is shown and you earn from valid clicks on those ads. “Google now completed a full rewrite of the link unit matching system. The new system is much smarter: it not only uses more sophisticated topic ranking algorithms but also continuously optimizes by learning which topics have the “most appeal” to visitors of your site. Results are overwhelmingly positive — topic quality, user response rate to topics and ads, and monetization metrics have all improved in significant, measurable ways,” states Google.