How Do Cookies Work?

The HTTP protocol used to exchange information files on the web is used to maintain the cookies.

There are two types of the HTTP protocol. Stateless HTTP and Stateful HTTP protocol. The stateless HTTP protocol does not keep any record of previously accessed web page history. While Stateful HTTP protocol does keep some history of previous web browser and web server interactions and this protocol is used by the cookies to maintain the user interactions.

Whenever a user visits a site or page that is using a cookie, the small code inside that HTML page (Generally a call to some language script to write the cookie like cookies in JAVAScript, PHP, Perl) writes a text file on users machine called a cookie.

Here is one example of the code that is used to write a Cookie and can be placed on any HTML page:

Set-Cookie: NAME=VALUE; expires=DATE; path=PATH; domain=DOMAIN_NAME;

When a user visits the same page or domain later time this cookie is read from disk and used to identify the second visit of the same user on that domain. The expiration time is set while writing the cookie. This time is decided by the application that is going to use the cookie.