Private enterprise is building what amounts to national data centers, gathering everything known about you. Say you've sought information about a debilitating disease, spent time in a chat room for recovering alcoholics, surfed porn sites at night or gambled online. Those Web sites you visit may be attached to your name, address, e-mail address or even phone number. They can implant a unique identification number, which tells a site exactly who you are when you return. When you buy a new computer and transfer your data from the old one, the cookies come along.
Cookies can make Web life easier.
For example, they remember your password when you revisit a useful site. Marketers, by contrast, use cookies to track your personal preferences, so they can sell you stuff. Often, those ads have been sent to you deliberately. Your cookie showed that you were at a particular site, and the marketer served you an ad for something your profile says you might like. When told about cookies, consumers tend to treat them as nothing more than a nuisance, like junk mail.