Session is a conversional state between client and server and it can consists of multiple request and response between client and server. Since HTTP and Web Server both are stateless, the only way to maintain a session is when some unique information about the session (session id) is passed between server and client in every request and response.
Some of the common ways of session management in servlets are:
User Authentication
HTML Hidden Field
Cookies
URL Rewriting
Session Management API
