It's called retargeting. It's a pretty effective way to market to people online. It allows ads you've recently seen to follow you around the net (making it more likely for you to buy).
Another really innovative marketing technique you've probably started to see is exit intent popups. The popup only happens when your mouse goes to close the tab.
There's a lot of cool shyt when it comes to marketing online.
I'm talking about if you visit a site. And then you go another site, Ads(advertising products from previous site) pop up in the current site you at.