These cookies are placed not only by the web site the consumer happens to be visiting, but also by third parties who may not be evident to the user. Cookies are used by the sites you visit to identify your computer. A cookie, for instance, enables an on-line bookstore to recognize your computer when you visit, to greet you personally, and to post a list of your favorite books to purchase. Cookies will not physically harm your computer, but they do stay on your hard drive until you choose to delete them.
Cookies can contain a variety of information.
Often, they contain a unique identifier, or UID, which tells the website which particular computer is visiting. Because a site can access the cookie information about that particular computer in the future, including the unique ID, that site can use the cookie to monitor the pages a user visits and other browsing behavior over time. Online advertising companies, which serve billions of graphic banner advertisements per day, are able to exchange cookie information with a consumers' computer as they deliver banner ads to a page a consumer is viewing, even if the consumer does not click on the banner ad. Because these online advertising services provide banner ads and other services to thousands of the most popular web sites, the companies can compile a profile of the pages a user visits across different web sites and over time. The exchange of cookie information and the profiling it enables take place without the knowledge or consent of most consumers.