The library is a unbundled static testing library for Android that containers all Android testing frameworks by Google. It will become a available through the Android Support repository and as open source in AOSP.

So what does it contain?

• AndroidJUnitRunner
  This Runner, based on android.test.InstrumentationTestRunner, runs JUnit3 and JUnit4 tests against an Android package (application). Let me give an example:
  @RunWith (JUnit4.class)
  public class MyJUnit4Test {
  {
    Droidcon mDroidconUK;
   
    @Before
    public void setUp() {
      mDroidconUK = new Droidcons.get(Droidcon.UK);
      mDroidconUK.open();
    }
  
    @Test
    public void checkPreconditions() {
      assertNotNull("mDroidconUK cannot be null", mDroidconUK);
    }
  
    @After
    public void tearDown() {
      mDroidconUK.close();
    }
  }

• Espresso
  Introduced at the GTAC in 2013, Espresso makes it possible to write concise, beautiful, and reliable Android UI tests quickly. Lets look at some examples:
  // Find view using a Matcher and type text using a ViewAction
  onView(withId(R.id.message_edit_text)).perform(typeText(TEXT_MESSAGE), closeSoftKeyboard());
  
  // Perform a click ViewAction
  onView(withId(R.id.send_button)).perform(click());
  
  // Verify using a ViewAssertion
  onView(withId(R.id.received_message_text_view)).check(matches(withText((TEXT_MESSAGE))));

• Intento
  Intento is like Mockito but for Intents. Basically a mock framework that allows you to create and configure mock objects. Let me give you an example:
  public void testDailerInput_typeNumberAndCall() {
   
    //Type phonenumber in dialer
    onView(withId(R.id.send_data_to_call_edit_text)).perform(scrollTo(), typeText("123-345-6789"), closeSoftKeyboard());
   
    // Click the call button
    onView(withId(R.id.send_to_call_button)).perform(scrollTo(), click());
   
    //Validate Intent has been send
    intended(allOf(
        hasAction(Intent.ACTION_CALL),
        toPackage("com.android.phone"),
        hasData(allOf(hasSchemaSpecificPart("tel", "123-345-6789")))),
        times(1));
  }

In conclusion: Testing with Android is finally becoming a more prominent part of Android Development cycle with the introduction of the Android Testing Support Library