Hi,
If you have read about MVC(Model View Controller) if not do read it. it will be good for understanding this..
Here Servlet is your controller that takes care of your request and response processing..
Your controller calls the model for business logic processing. and posts the reponse back to the client.
Model can be your EJB, or a webservice, or a plane
java call.
So EJBs are used as model in distributed systems (where your different components stay on different machines).. like your servlet can on one server and ejb can be deployed on different server.
And view is your display can be a
jsp or plane html..