Remarketing on the Google Display Network (GDN) with Google AdWords:
With remarketing, you can reach customers who have shown an interest in your product or service by visiting your site, and show them relevant ads across the Google Display Network. It's a powerful way to match the right people with the right message.

How remarketing works
A recent change to remarketing has made implementation far more simple. Previously, you had to enter separate piece of code (also called a ‘tag’) for each category that you may have wanted to track for later promotion, but you can now install a single tag across every page of your website, and then create audiences within AdWords that you want to track, even using combinations of audiences to later promote specific advertisements to. For example, you can create a “TV” audience to track visitors to all of the pages where you sell TVs, for example, as well as a "completed order" audience to track visitors who have in fact made a purchase, and you can later display highly relevant and engaging ads, perhaps including special offers or discounts, to users who have visited your tv pages, but didn’t make a purchase.