Cookies are plain text files, so they can be easily viewed by any text editor or word processor. Each cookie contains a complex string of characters and the address of the issuing site. These numbers identify the cookie to the server and match you to the record of your previous visit.
The ability to detect what operating system you are running, your browser version and from which site you arrived at the page are all contained in a cookie. Page preferences, the types of information you want to see, a password, your name can all be stored in a cookie. More sophisticated cookies can detect your e-mail address, if entered into the browser, plus your IP address and domain name.
Cookies can be used by web sites to track how often pages are visited, and by whom, as well as the route visitors take to get there. Thus you can receive several cookies from a single site, each targeted at a specific piece of information. Site login information and individual user preferences are all activated via a cookie.
When you shop online, the tracking of potential purchases as you add to a shopping basket, is accomplished with a cookie.
Cookies are also the mechanism that advertisers use to, among other things, direct marketing... what advertisements you see and with what frequency, when you visit a site.