Think about the Web sites you visited recently. You shopped, checked airfare prices and researched a medical condition.

How would you like marketers to trade this information, along with your name and address? They may very well have.

Increasingly, online tracking is becoming more personal. Cookies used to be relatively harmless. Now, cookies can tie your surfing habits to your offline identity.

How cookies work

A cookie is a small text file placed on your computer by a Web site. Some cookies are useful. They identify you to your favorite sites. With them, you can bypass site log-ins. The site remembers your preferences.

But advertisers and third parties also place cookies on your machine. For instance, you may see an Acme Widgets ad on one site. The ad places a cookie on your computer.

You visit a second site running an Acme ad. This time, Acme reads the cookie on your machine. It knows the content of the first page you visited. It knows the content of the second page you visited. It can offer an ad better targeted to your interests than the first ad.

Now, consider this. Acme ads are placed on sites via an advertising network. Goliath Advertising tracks and profiles you as you move from site to site. And it displays ads based on your interests.