This beautiful infographic is from MDG Advertising.
What is Retargeting?
Briefly, retargeting is advertising that targets people who have visited your website. For example, an ad promoting a petition might be displayed to a person who recently visited Change.org. These ads take advantage of recency and relevancy to get more conversions.
It emphasizes how Facebook News Feed ads have been welcome news for advertisers seeking a new way to effectively use retargeting.