Passage: Referrers; The host can track the (apparent) source of the links and determine which sites are generating the most traffic for a particular page.Websites produce traffic rankings and statistics based on those people who access the sites while using their toolbars and other means of online measurements. The difficulty with this is that it does not look at the complete traffic picture for a site. Large sites usually hire the services of companies such as the Nielsen NetRatings or Quantcast, but their reports are available only by subscription.

Question: Whats the name of the thing that tracks links?
Referrers