title: What is Ruby's ThreadGroup for?
body: <p>I was flicking through the Pickaxe, looking for the documentation on <code>Thread</code>, and came across <code>ThreadGroup</code>.</p>
  
  <p>The documentation describes what it does, but it doesn't explain what it's for.</p>
  
  <p>Is a thread group related to a thread pool, which I <a href="https://stackoverflow.com/questions/3840751/why-doesnt-ruby-have-a-threadpool-built-in">assumed Ruby doesn't have</a>?</p>