One of the fastest-growing online businesses is the business of spying on Internet users. Using sophisticated software that tracks people's online movements through the Web, companies collect the information and sell it to advertisers.

Every time you click a link, fill out a form or visit a website, advertisers are working to collect personal information about you, says Joseph Turow, a professor at the Annenberg School for Communication at the University of Pennsylvania. They then target ads to you based on that information.

On Wednesday's Fresh Air, Turow — the author of the book The Daily You: How the New Advertising Industry is Defining Your Identity and Your Worth — details how companies are tracking people through their computers and cellphones in order to personalize the ads they see.