what is loading a javascript code asynchronously
<p>I have seen facebook javascript api mentioning that loading the sdk asynchronously </p>

<pre><code>// Load the SDK Asynchronously
  (function(d){
     var js, id = 'facebook-jssdk'; if (d.getElementById(id)) {return;}
     js = d.createElement('script'); js.id = id; js.async = true;
     js.src = "//connect.facebook.net/en_US/all.js";
     d.getElementsByTagName('head')[0].appendChild(js);
   }(document));
</code></pre>

<p>what is the difference in loading a script using the script tag directly like the following</p>

<pre><code>&lt;script src='as.js'&gt;&lt;/script&gt;
</code></pre>

<p>and the above shown code</p>
