On the Net and off, personal data is a currency, an entity that can be bought, sold, bartered and, yes, stolen. Ideally, this information connects companies with potential clients and consumers with products and services. Ads with the precision of surgical airstrikes are swell for advertisers, but on the flip side are personalized services that can actually make online life easier. If the maitre d' knows your face and your preferences, he can make sure that your meal will be enjoyable. Because Web sites can't see your face (and wouldn't recognize it even if they did), they employ various methods of recognition. One of these is known as a cookie file.

If you haven't heard of them, here's a quick way to see them at work. Go to your Web browser's preferences/options, look for the "cookies" section and change the setting to "Notify before accepting." Now visit a few major sites and get ready to keep whacking the "Accept" (or "Decline") button.

Cookies are now embedded everywhere. You sometimes have to gobble up several before you even get past the front door. The Web is, by design, in constant flux; cookies are tags that try to pin you down. They have many uses, and not all of them are nefarious. For example, certain cookies for news sites such as The New York Times eliminate the need to submit your password each time you enter the site. And virtual shopping carts couldn't move through online aisles without cookies.