title: What is the difference between a synchronous and an asynchronous request? (async=true/false)
body: <p>What is the difference between <code>async=false</code> and <code>async=true</code>, when I am using <code>open</code> method of <code>XMLHttpRequest</code>?</p>
  
  <pre><code>function GetXML() {
  
      if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
          xmlhttp = new XMLHttpRequest();
      }
      else {// code for IE6, IE5
          xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
      }
      xmlhttp.onreadystatechange = test
      xmlhttp.open("GET", "PlanetCafe.xml", true);
      xmlhttp.send(null);
  }
  </code></pre>