Retargeting is a form of online targeted advertising in which the advertising being displayed to a visitor is targeted based on previous Internet actions. These actions include search engine inquiries, website visits, previous clicks on promoted links or ads, etc. that didn't result in a call-to-action being completed.
Most known retargeting networks are Google AdWords/AdSense, Facebook, Criteo and AdRoll. Retargeting is often referred to as Remarketing.