How does referral targeting work?

Referral targeting uses a HTTP field called HTTP referer (yep, incorrectly spelt!) which identifies the address of the webpage that linked your visitor to the webpage that's now being requested (your website). By checking the referer field, we can see where the visitor originally came from.
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Referer URLs are logged by many web analytics programs including Google Analytics. You can monitor these URLs to get insight into where your web traffic is coming from.