title: What is the concept of application, job, stage and task in spark?
body: <p>Is my understanding right? </p>
  
  <ol>
  <li><p>Application:
  one spark submit. </p></li>
  <li><p>job:
  once a lazy evaluation happens, there is a job. </p></li>
  <li><p>stage:
  It is related to the shuffle and the transformation type. 
  It is hard for me to understand the boundary of the stage. </p></li>
  <li><p>task:
  It is unit operation. One transformation per task. One task per transformation. </p></li>
  </ol>
  
  <p>Help wanted to improve this understanding. </p>