If you get a deja vu feeling while surfing the web these days it's very likely due to somebody retargeting you. Retargeting is a web advertising technology which is very effective in breaking through banner blindness that most regular web users developed over the years of bombardment with banner ads. Retargeting works by marking a user visiting advertiser's web page and then shooting her with excessive amount of banner ads reminding the user of advertiser's offering. The marking of the user is usually implemented through cookies and there are specialized companies (e.g. AdRoll or SiteScout) that make sure that the marked user receives the right advertising.
Initially, retargeting was mostly about behavioral targeting since a user placing items in her shopping cart is very likely in the process of buying something and shooting such user with a discount code or some other stimulant might just tip her over and close the sale. But increasingly retargeting is used also for user segmentation as readers of Cars.com, for example, are much more likely to be considering buying a new car than readers of ArianaGrande.com (unless they are fathers helping their 10 years old daughters fill their iPods).
Retargeting can easily become creepy since it's an obvious sign that your every step on the web is monitored by at least a dozen of companies. So I expect quite some user revolt and involvement of politics in the near future. But until stricter regulations kick in the retargeting is extremely efficient selling technique that makes some companies (e.g. Amazon.com) millions of dollars and which is here to stay in one way or another regardless of any policy intervention.