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Cookie
Cookie - A small text file on the user’s PC that identifies the user’s browser and hence, the user so they are ‘recognised’ when they re-visit a site; eg, it allows usernames to be stored and websites to personalise their offering. Cookies are specific to a particular domain (the domain of the server the user is visiting when the cookie is created), and generally have a specified "lifetime" after which they are invalidated. They are commonly used to store "state" information.
Web-browser applications are inherently "stateless" - which means that between one page request and the next, no information is exchanged between the browser and the server. Note - this is changing with the increasing use of AJAX to provide a way for a browser to communicate with a server "asynchronously".
In order for a server to "keep track" of a page request and determine that it is coming from the same user as a previous request, it needs to store information on the browser to identify that user. This can be done in two ways - either by using a cookie to store the information or by encoding the information in the URL for the next request. Usually the technique used is to encode a "session ID" identifying that unique user in either the cookie or the URL. The session ID is then used by the server to lookup information it has stored specific to that unique browser (such as what page request was made previously).
Cookies can also be used by third-party measurement or ad-serving companies to track a user's activity across many sites, since the domain used by the 3rd-party server is the same across many sites.
Users can delete cookies stored by their browsers, either manually or by using browser features or "cookie crusher" software. They can also set their browser security to disallow cookies - either from all sites or from specific sites. If they do this, sites that rely on cookies to operate will not work properly unless they have been built to allow an alternative way of tracking users (such as the URL encoding method mentioned above).
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