Cookies are piece of data which are stored at the client?s browser to track information regarding the user usage and habits. Servlets sends cookies to the browser client using the HTTP response headers. When client gets the cookie information it?s stored by the browser in the client?s hard disk. Similarly client returns the cookie information using the HTTP request headers. When cookie based session management is used, a token is generated which contains user's information, is sent to the browser by the server. The cookie is sent back to the server when the user sends a new request. By this cookie, the server is able to identify the user. In this way the session is maintained. Cookie is nothing but a name- value pair, which is stored on the client machine. By default the cookie is implemented in most of the browsers.

For more information, visit the following links:

http://www.roseindia.net/jsp/jspcookies.shtml

http://www.roseindia.net/servlets/use-cookie-servlet.shtml