title: Use and meaning of session and process group in Unix?
body: <p>Unix processes have a <em>session id</em> and are part of a <em>process group</em> - which can be changed/queried with functions such as <code>setsid()</code>/<code>getpgrp()</code>.</p>
  <p>However the concept of a <em>process group</em> and <em>session</em> always eluded me, could anybody explain what significance having distinct sessions and process groups provide - why/when do one want to create a new session or place several processes in the same session and/or process group ?</p>