Can someone please tell me the difference between these in plain english? I cannot find a clear distinction between them, dispite having reading that chapter several times.

Is an EntityManager a PersistenceContext?

"A persistence unit maps a fixed set of classes to a relational database."
This is a quote from the Enterprise Javabeans 3.0 book.

So what's the difference between a PersistenceUnit and a PersistenceContext?

I'm particularly interesting in learning when I would use different configurations of these and when to use a particular configuration For example, would I have one EntityManager with two PersistenceContexts because I need to manage my entity objects across two databases?

Thanks for any help, Michael.