Retargeting, also called remarketing, begins by dropping a cookie on site visitors. This enables marketers to follow them as they search, do social networking or flit from site to site. A person coming to your site can then be discretely followed by ads reflecting the content they viewed or reflecting their psycho-demographic profile. 

If you watch Mad Men regularly and filter out all the illegal and non-PC stuff, it becomes pretty clear that not much has changed in Adland in sixty years. Agencies are run pretty much the way they are depicted on TV. Evidently the great management and technology revolutions sidestepped Madison Avenue.