title: Difference between Firebase AngularFire implicit and explicit sync
body: <p>I am new to firebase. I want to build it with angularjs and I found angularfire.</p>
  
  <p>In the angularFire docs listed
  Implicit and explicit sync. I tried to understand the document in github but I still don't understand what is the difference and how to use them.
  <code>angularFire()</code> and <code>angularFireCollection()</code></p>
  
  <p>also, what are the arguments mean in <code>angularFire()</code> and <code>angularFireCollection()</code>?</p>
  
  <p>thank in adv</p>