jQuery: What's the meaning of comparison in following code block - callback && function() {callback.call();}
<p>Seeing this block from <code>jQuery.scrollTo.js</code> library (in <a href="http://flesler-plugins.googlecode.com/files/jquery.scrollTo-1.4.2.js" rel="nofollow">v1.4</a> at line 184).</p>

<pre><code>function animate( callback ){
    $elem.animate( attr, duration, settings.easing, callback &amp;&amp; function(){
        callback.call(this, target, settings);
    });
};
</code></pre>

<p>Curious to know how the comparison is going to work with </p>

<pre><code>callback &amp;&amp; function() {callback.call(...)}; 
</code></pre>

<p>and what's exactly the meaning behind this. Thanks in advance.</p>
