Cookies

In HTTP communication, a cookie is a single piece of information with name, value, and some behavior parameters stored by the server in the client's filesystem or web browser's memory. Cookies are the de facto standard mechanism through which the session ID is passed back and forth between the client and the web server. When using cookies, the server assigns the client a unique ID by setting the Set-Cookie field in the HTTP response header. When the client receives the header, it will store the value of the cookie; that is, the session ID within a local file or the browser's memory, and it will associate it with the website URL that sent it. When a user revisits the original website, the browser will send the cookie value across, identifying the user.

Besides session tracking, cookies can also be used to store preferences information for the end client, such as language and other configuration options that will persist among sessions.