Remarketing refers to Google’s particular brand of re-targeting. Remarketing is when you target a custom ad to someone based on an action they took in the past. The most simple example of remarketing is when you show ads to people who have visited your website in the last 90 days. It works by placing a “cookie” in someone’s browser with they visit your website. Then, when that same person is reading an article on the New York Times or Washington Post (both of which are Google advertising partners in the Google Display Network), they might see an ad for your business.
Because you are able to target people based on actions they have taken, remarketing can be highly effective. For example, you can show an ad to someone who has started to register for an event, and your ad can specifically prompt the person to finish their registration.