Transaction state on connection Idle/Open/Failed

In JDBC (latest version), I'm using the 
org.postgresql.jdbc.PgConnection.getTransactionState() that returns an 
enumeration of IDLE, OPEN or FAILED.

I am familiar with IDLE, meaning the connection has a new transaction 
started, but isn't doing anything.  We think of this as the "normal" 
state for our connections.

How does OPEN compare to IDLE?  If we stop using a connection that's in 
the OPEN state, is something wrong?  That is, does it have pending 
updates that require a commit/rollback?

Same for FAILED.  If we no longer need a connection in the FAILED state, 
is something wrong?  Does it have pending updates that require a 
commit/rollback (assuming in this state rollback makes more sense).

I'm really trying to find out what it means for the connection to be in 
OPEN or FAILED states compared to IDLE.

Thanks,
David