As your applications grow more complex, you'll need to keep better track of which user your program is working with. Cookies, sessions, and access control all provide an opportunity to interact more appropriately with specific users.
You can track certain user details like the number of visits, names, or the date of the last visit using cookies, which are small bits of text stored on the client that have been available since Netscape 1.0. The client machine stores this information and sends it to the web server whenever there is a request. Cookies data is sent along with the HTTP headers.
After the first visit to a web site, the browser returns a copy of the cookie to the server each time it connects. For security reasons, cookies can be read only from the domain that created them. Additionally, cookies have an expiration date after which they're deleted. The maximum size of data that a cookie can hold is 4 KB.