Retaining Client and State Information
All but very simple Web applications are likely to require that information of some kind about the client be retained between different page requests. As has been stated, HTTP is stateless and does not provide a mechanism to ascertain that a series of requests have come from the same client.
There are a number of ways of retaining information about clients, such as hidden fields, cookies, and sessions (all described in this section). Sessions are by far the simplest to use, but you should be aware that no one method provides a perfect mechanism, and your servlet may need to be written to support more than one technique.
Using Session Objects
Sessions identify page requests that originate from the same browser ...