Do selectors make a copy, or do they point to the actual element?

Do selectors make a copy, or do they point to the actual element?

 I have the following code:
  1.           var elementList = document.querySelectorAll(".dotImageCommentClass");
                        top =  elementList[0].offset().top;
             

A diagnostic alert tells me I have an image (or array of them) but I get an error that the 'offset' property is not supported!
Why is that?

Thanks.