title: What do dot and hash symbols mean in JQuery?
body: <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>