This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I am going to guess that you are talking about the fact that a servlet container creates a single instance of a servlet class and that instance may persist in memory for an indefinate amount of time and service any number of requests.
Whenever u instantiate a servlet, only a single servlet instance persists in the web container for processing your requests. All your requests to this servlets is handled by the single instance of that servlet. This servlet is removed form your web container only when your application server is brought down or on redeployment.