In my time of digging around Java APIs I have come across both impl and internal packages. Up until now I never really thought about the difference - as with all enterprisey Java apps, I figured they just meant that "actual implementation in here; you (API user) should be really using the interface. Go away."

A little bit of digging around Stack Overflow seems to suggest that the internal package at least can have some security placed around it.

So, what is the difference? I don't think it is a matter of taste because I have seen APIs with both.