What do dot and hash symbols mean in JQuery?

Question

I feel confused of the dot and hash symbols in the following example:

<DIV ID="row">
<DIV ID="c1">              
<Input type="radio" name="testing" id="testing" VALUE="1">testing1
</DIV>
</DIV> 

Code 1:

 $('#row DIV').mouseover(function(){     
    $('#row DIV').addClass('testing');  
    });

Code 2

  $('.row div').mouseover(function(){
        $(this).addClass('testing');
    });?

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

Answer 1

The hash (#) specifies to select elements by their ID's

The dot (.) specifies to select elements by their classname

You can read more about the selectors here: http://api.jquery.com/category/selectors/basic-css-selectors/