One of the fastest-growing online businesses is the business of spying on Internet users by using sophisticated software to track movements through the Web, so that the information can be sold to advertisers. Julia Angwin recently led a team of reporters from The Wall Street Journal in analyzing the tracking software. They discovered that nearly all of the most commonly visited websites gather information in real time about the behavior of online users. The Journal series identified more than 100 tracking companies, data brokers and advertising networks collecting data - which are then sold on a stock market-like exchange to online advertisers. Angwin explains how consumer surveillance works, how users can disable the tracking software - and how advertisers are continually evolving to keep up with the data they receive. She notes that many Internet users are unaware that their information is being tracked and then traded. Most people that we have heard from since writing these stories did not know what was going on," Angwin explains. "So when you go to a website, you're not thinking about the fact that they might have relationships with all different types of monitoring firms, and those firms are installing things that are invisible to you on your computer."