It's the difference between "the rug" and "the frayed Middle Eastern rug with a faded black and red diamond patter"; it's the difference between "the tree" and "the tall pine, a hundred feet if not more, full of summer green".
Kristine Kathryn Rusch posted, on her blog, an essay she wrote about the difference between "real details" and "fake details" with examples (much better than my poor examples above). Highly recommended reading, available on her blog.
