A cookie is a little bit of text stored by a user's web browser. Cookies can be used for authentication, the identifier for a server session, storing site preferences, shopping cart contents, or just about anything else that can be done with text data. A cookie is made up of one or more name-value pairs containing bits of information. The cookie is sent as an HTTP header by a web server to a web browser and then sent back unchanged by the browser each time it accesses that server.
Although cookies can provide a useful function, some may not be so desirable. As the Internet evolved from its beginnings in government and academia to more commercial purposes, cookies are also used as tracking mechanism used by advertisers. The Wall Street Journal has published a new report The Web's New Gold Mine: Your Secrets. Anyone interested in online privacy will find this very interesting. Below is a video which is part of this report: