Servlet is one of the Java technologies which is used for developing server side web application.
There are two types of Java Servlet Generic and HTTP Servlet.
While going through an interview you could be asked for number of question on Servlet, that can be simple or complex one depend on your experience.
An interviewer can ask you for anything either it's a difference between generic and HTTPServlet or life cycle of Servlet. So just prepare yourself according to your experience.
And for collection of Java Servlet Interview Question you can visit here at http://www.roseindia.net/interviewquestions/servlet/index.shtml