The World Wide Web was designed in such a fashion that all the web servers would process the URL requests in the similar way without any dependence on the underlying operating systems on which they were running. Similarly browsers were also required to send URL requests to servers. Hidden form fields were introduced first, followed by the HTTP cookies. These allow CGI programs to maintain information about individual web browsers.
These fields are not visible and hence, the user cannot modify them. Forms that contain hidden fields may be generated dynamically by java script.
When a server sends a form to a browser it sets the value of a hidden field. When the user enters data in a form and submits the same, the original value stored in the field is returned to the server, and the server uses this information to know about the state of the browser. Consider an example where you are asked to fill in the form fields, and the form has the text field for your e-mail address. Once you send this form it sends back another form, which already contains your email address that you had supplied in the earlier form. It also stores this information in a hidden field, which is used by the server to relate the two forms coming from the same browser.
Hidden form fields do not maintain the browser state in the persistent manner, which means that the state of browser is lost between the sessions. Cookies were developed to answer this problem. Cookies maintain browser state information in a text file which is also termed as cookie jar. A server side program sends this cookie file when a user sends a URL request. When a browser sends a URL request , the browser first searches cookies.txt file for any matching cookies. Browsers can also send the information stored in cookies if the server requests for the same.
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