Difference between Android Instrumentation test and Unit test in Android Studio?

As of Android Studio 1.1rc there's Unit testing support and I'm wondering what's the difference between Android Instrumentation Tests and Unit tests.

As I understand it:
Unit tests are useful for testing code which doesn't call the Android API, and the Android instrumentation tests are rather integration tests to test Android API specific elements or GUI components.

However if you use a framework like Robolectric or Mockito in your unit tests, you can test Android code (without the need of a device) if I'm not mistaken.

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Is this correct, or is there a bigger difference? If so, what's the use of each?