What do dot and hash symbols mean in JQuery
<p>I feel confused of the dot and hash symbols in the following example:</p>

<pre><code>&lt;DIV ID="row"&gt;
&lt;DIV ID="c1"&gt;              
&lt;Input type="radio" name="testing" id="testing" VALUE="1"&gt;testing1
&lt;/DIV&gt;
&lt;/DIV&gt; 
</code></pre>

<p>Code 1:<br/></p>

<pre><code> $('#row DIV').mouseover(function(){     
    $('#row DIV').addClass('testing');  
    });
</code></pre>

<p>Code 2<br/></p>

<pre><code>  $('.row div').mouseover(function(){
        $(this).addClass('testing');
    });​
</code></pre>

<p>Codes 1 and 2 look very similar, and so it makes me so confused that<br/>
when I should use ".row div" to refer to a specific DIV instead of using "#row div" ?</p>
