title: difference between async.series and async.parallel
body: <p>What is the difference between async.series and async.parallel. Consider the following exmaple, and i've got the same result.</p>
  
  <pre><code>async.parallel([
      function(callback){
          setTimeout(function(){
              callback(null, 'one');
          }, 200);
      },
      function(callback){
          setTimeout(function(){
              callback(null, 'two');
          }, 100);
      },
      function(callback){
          setTimeout(function(){
              var err = new Error('I am the error');
              callback(err);
          }, 400);
      },
      function(callback){
          setTimeout(function(){
              callback(null, 'three');
          }, 600);
      },
  ],
  // optional callback
  function(err, results){
      if(err){
          console.log('Error');
      } else {
  
      }
      console.log(results);
      //results is now equal to [ 'one', 'two', undefined ]
      // the second function had a shorter timeout.
  });
  </code></pre>
  
  <p>and </p>
  
  <pre><code>async.series([
     function(callback){
          setTimeout(function(){
              callback(null, 'one');
          }, 200);
      },
      function(callback){
          setTimeout(function(){
              callback(null, 'two');
          }, 100);
      },
      function(callback){
          setTimeout(function(){
              var err = new Error('I am the error');
              callback(err);
          }, 400);
      },
      function(callback){
          setTimeout(function(){
              callback(null, 'three');
          }, 600);
      }
  ],
  // optional callback
  function(err, results){
      //results is now equal to [ 'one', 'two', undefined ]
      if(err){
          console.log('Error');
      } else {
  
      }
      console.log(results);
  });
  </code></pre>
  
  <p>i don't see the difference. Maybe is my sample bad? I read the documentation about these two function on github async repository, you can find for async.parallel function:</p>
  
  <blockquote>
    <p>If any of the functions pass an error to its callback, the main
    callback is immediately called with the value of the error</p>
  </blockquote>
  
  <p>what is the main callback in async.parallel?</p>