Site retargeting is a display advertising technique used by marketers to display advertising to people who have previously visited their website. The marketer includes a pixel within their webpage which sets a cookie in the user's browser. That cookie allows the marketer to target the website visitor with advertising elsewhere on the internet using retargeting.
A related technique used by retailers involves dynamic creative, which allows the marketer to customize the ad displayed to any site visitor, for example displaying a picture of a product the visitor was viewing earlier, but did not purchase.
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Site retargeting Wikipedia(Text) CC BY-SA