What is the difference between a session and a transaction in JPA 2.0?

You go to the bank to deposit 2 checks, and withdraw a small sum.

So you stand in line until a teller opens.

You make your first deposit.
Then your second.
Then you make your withdrawal.

Now you're done, you leave the teller line.

Getting to the teller is like creating your session, now you're in the bank, ready to work.

Each deposit and withdrawal are their own contained set of work, these are your transactions.

When you're done with your work and leave, you're closing or abandoning your session.

So, in essence, a session contains your transactions, after all you can't make a bank deposit if you never go to the bank right?

em = SessionFactory.startSession();

In JPA, there is no Session and no SessionFactory. SessionFactory is a hibernate-specific interface that you shouldn't use if you use JPA (use either Hibernate's own API or use Hibernate as JPA Provider, but not both.)

A session is what you use to interact with the database.

A transaction is used to specify boundaries for the session to operate within.

Essentially, transactions prevent the database from being corrupted by only allowing a session to interact with it at one time. (It's a bit more complicated then that, as you can have many transactions reading from the database, but only one transaction that's writing.)