title: What's the difference between the "global queue" and the "main queue" in GCD?
body: <p>Among some other ways, there are these two ways to get queues in <code>GCD</code>:</p>
  
  <pre><code>dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
  
  dispatch_get_main_queue();
  </code></pre>
  
  <p>If I'm not completely wrong, the "main queue" is executing on the main thread and is good for "callback" blocks which execute UI work.</p>
  
  <p>Does this mean a "global queue" is one that runs on a background thread?</p>