title: Reference vs pid?
body: <p>I'm not entirely sure the differences between the PID and Reference and when to use which.</p>
  
  <p>If I were to spawn a new process with <code>spawn/1</code> pid. I can kill it with the PID no? Why would I need a reference?</p>
  
  <p>Likewise I see <code>monitor/1</code> receiving a message with a ref and pid number.</p>
  
  <p>Thanks!</p>