Remarketing

Remarketing allows Google AdWords/AdSense advertisers to target people who previously visited their website (most of which did NOT make a purchase) with additional and more specific ads as the visitor browses OTHER sites in the Google Display Network.

Google provides this example: As online shoppers look for new running shoes, they visit a popular online store, FastSneakers.com, to browse the different styles. However, many users may leave without buying anything. FastSneakers.com could add these users to a "Site Visitor" list. This will enable FastSneakers.com to reach out to these potential buyers, as they browse other websites, with a compelling call-to-action or offer that will encourage them to return to the website to complete a purchase.