Ad companies have their hands, or cookies, in your browser

Your computer's appetite for cookies would put the Cookie Monster to shame. Without letting you know, your Internet browser gulps down hundreds of the digital tracking beacons fed to it by the websites you visit, sometimes storing them for months or years, enabling an array of companies you've never heard of to monitor what you do online.

Cookies are not inherently dangerous — they are a kind of virtual ID card that helps browsers perform a number of online tasks that most users have come to expect: allowing you to stay logged in to your Google or Yahoo email service, or to go from page to page on Amazon.com while filling up your shopping basket with books. Each time you add a new book, for instance, your browser sends a request to Amazon's servers, flashing the special ID code on the cookie. Because Amazon placed the cookie on your computer in the first place, it will recognize the ID it assigned to you, look up your account and add the new book to your basket. But the useful cookies are far outnumbered by those that serve no other purpose than to keep track of what you do online. They often come from sites you have never visited. This can happen when you open pages that carry embedded advertisements, which are capable of adding cookies to your browser without your permission or knowledge — and often do.