title: what is a Callback mechanism and how it applies in R
body: <p>Can someone explain to me, in simpler non-computer science term, what is the Callback features in R (<code>taskCallback</code>, <code>getTaskCallbackNames</code>,  <code>taskCallbackManager</code> ... etc)? I have looked at the R help, but I find the information a bit too abstract.</p>
  
  <p>What is it design to do, and how can a user make use of them?</p>
  
  <p>If someone can explain the general concept (in R and other computer languages), and provide example in R, I would appreciated very much, since I never really understood. Does it have to do anything with recursive functions, or am I misguided by the name callback?</p>