You should only have one root folder because it makes it much easier and faster to upload and maintain your site.
You need an images folder so your graphics are easily accessible and ensure that your images don't collide with your .htm's or .html's.
An absolute link is one that is accessible from any web page because of its URL. A relative link is one in which it is only linked to one specific document. It is beneficial for the site maker to use relative links because they link faster and are easier to make.
