title: Difference between support-v4.app.fragment and app.fragment?
body: <p>I am new to fragments and had a question:
  When I updated my SDK tool and went to create a project(min = api8, the default one), I noticed the android-support-v4.jar in my Referenced Library.</p>
  
  <p>When I extended my class and tried to use the <code>Fragment</code> class, I found out there are 2. One is android.app.Fragment and other is android.support.v4.app.Fragment. What is the difference between these two?</p>