How Do They Track You? Let Us Count the Ways - Bits - Technology - New York Times Blog
On the Internet, companies are typically ranked by how many different people visit their sites in a given month. And when Microsoft announced its $41 billion bid for Yahoo, comScore and Nielsen Online promptly put out estimates counting how many people would be in the merged company’s total audience.
But audience size is not everything in the online world. Advertisers increasingly want media companies to find their most likely customers and show their ads only to those people, rather than to the site’s entire audience.
From Internet Marketing