what is the difference between `createActivity` and `shadowOf`?

I saw this roboletirc example:

 @Test
  public void shouldHaveHappySmiles() throws Exception {
    String hello = new MainActivity().getResources().getString(R.string.hello_world);
    assertThat(hello, equalTo("Hello world!"));
  }

  @Before
  public void setup()  {
    activity = Robolectric.buildActivity(MainActivity.class)
        .create().get();
  }
  @Test
  public void checkActivityNotNull() throws Exception {
    assertNotNull(activity);
  }

  @Test
  public void buttonClickShouldStartNewActivity() throws Exception 
  {
      Button button = (Button) activity.findViewById(R.id.button2);
      button.performClick();
          Intent intent = Robolectric.shadowOf(activity).peekNextStartedActivity();
          assertEquals(SecondActivity.class.getCanonicalName(), inten

t.getComponent().getClassName());
  }

I tried to look in documentation but didn't understand the difference between createActivity and shadowOf. can someone please shed some light?

btw, why is there a new MainActivity().getResources().getString(R.string.hello_world); in this test? when should I use new instead of createActivity() ?