Marketers are spying on Internet users -- observing and remembering people's clicks, and building and selling detailed dossiers of their activities and interests. The Wall Street Journal's What They Know series documents the new, cutting-edge uses of this Internet-tracking technology. The Journal analyzed the tracking files installed on people's computers by the 50 most popular U.S. websites, plus WSJ.com. The Journal also built an "exposure index" -- to determine the degree to which each site exposes visitors to monitoring -- by studying the tracking technologies they install and the privacy policies that guide their use.