Retargeting:

Behavioral retargeting (also known as behavioral remarketing, or simply, retargeting) is a form of online targeted advertising by which online advertising is targeted to consumers based on their previous Internet actions. Retargeting tags online users by including a pixel within the target webpage or email, which sets a cookie in the user’s browser. Once the cookie is set, the advertiser is able to show display ads to that user elsewhere on the internet via an ad exchange.