It is a static block of information. When a browser reconnects to a Web site that sent it a cookie, the cookie is sent back to the site, and the information is dissected and interpreted by whatever server program receives it. Cookies can be defined by their originating sites to be sent to a particular domain or subdomain and set of pages. If a particular cookie is not explicitly defined to be sent to a particular domain, it is sent by default only to the server and page where the cookie originated. In either case, sending a cookie is a proactive action by a Web browser, and thus can't be used by unauthorized sites to see where a user has been.
Cookies have lifespans that are defined by their expiration dates.
If a cookie has no expiration date explicitly defined, it is available to the browser for the current session only and will not persist. They are used to maintain a particular configuration or profile for a particular browser when connecting to a Web site. Some sites, such as the Microsoft Developer Network, use cookies to maintain a browser's current position in a set of pages. Other sites may keep track of things you've shown interest in before when visiting their site, or purchase choices in the case of on-line shopping. Some sites will not allow some pages to be viewed without first accepting a cookie.