Open Hibernate Question

Open question: I see in the Hibernate documentation that Hibernate's EntityManager is "similar to" Hibernate's Session interface. But it isn't clear when I should be using one versus the other. I'm using the EntityManager now and everything's going fine. But I want to make sure I'm doing this The Right Way so that I don't have to rip it apart and rewrite it in three months when I better understand the business problems I'm fighting. Any readers have any insight?