A cookie is basically a string of text characters not longer more than 4 KB. Cookies are set in the form of name/value pairs, separated by semi-colons.
Cookies has some limitation, some browsers will have a limit to how many cookies they can store, usually 300 cookies or more, of which there may be 20 cookies per domain name. A total of 4 KB (after encoding) of cookies can be stored for any domain or path.
A Cookie is text information that is transferred from the server to your Browser via the HTTP protocol and then stored on your hard drive. If a site uses a Cookie then everytime you visit that site, if the browser has a Cookie stored on it, it transfers that Cookie to that site. The program on the site using the Cookie then access the information and acts upon it. It is important to understand here that a Cookie will only be transfered to the site that created it. Part of the Cookie is the domain name of the site that created it as well as the path to the program that access it. Unless the domain and path match exactly as they were set, the Cookie IS NOT TRANSFERED! There is no way for someone from one site to see the contents of the Cookie from another site.