Google AdWords Remarketing is another really powerful feature. It allows you to reach (advertise) to people who have already visited your website or used your app.
When someone visits a particular page or pages of your website they are get added to a your remarketing list.
Then you have an AdWords Display network advertising campaign running that shows ads to only those people who are on your list.
They then see advertisements that refresh them about the product or service they were viewing on your website.