Imagine that when you walk into a mall, someone straps a device to your arm that tracks your shopping habits and other vital information. Each store you enter uses the device to learn about your purchasing behavior -- for example, what items you examined and rejected -- and maybe even to obtain personal information about how old you are and where you live. Would you accept this sort of shopping experience? If you use the Internet, it's a moot question; you already are being tracked this way.

Most online shops already use a tracking technology known as "cookies." This is computer slang for data that is stored on your hard disk so that a website will recognize you next time you arrive. You supply the information in each cookie -- usually by completing a registration form at the commerce site. The site then sends this information back to your computer for storage and later retrieval.

The information placed in a cookie is not only useful in the context of e-commerce. It can also enable you to receive a customized home page that, for example, supplies entertainment news, but not sports. Finally, cookies provide marketing information; they can track the ads you click on in order to provide you with similar banner ads in the future.