title: dismissViewControllerAnimated VS popViewControllerAnimated
body: <p>I want to know what is the different of them. 
  when I can call</p>
  
  <pre><code>[self dismissViewControllerAnimated:YES completion:nil];
  </code></pre>
  
  <p>and when I should call</p>
  
  <pre><code>[self.navigationController popViewControllerAnimated:YES];
  </code></pre>
  
  <p>according document of apple:
  dismissViewControllerAnimated means
  "Dismisses the view controller that was presented by the receiver."
  But I always fail to dismiss view controller by this method.</p>