How is $('<img/>') different form $('img')

How is $('<img/>') different form $('img')

I noticed this selector  $('<img/>')
I am wondering what is it really selecting? Is that different from the tag selector $('img')?

And a piece of code:


      var $img = $(this);

      //some function



Why does it have to find its own " src " and give it to itself again?

I appreciate if some can answer that for me.