Annette Lyon about specificity on the Writing on the Wall blog that helped clarify my understanding of concrete details:
"Showing has several elements, but specificity is one of my favorites. The gist is to take a general noun (such as a car) and tell us more. Make us see it.[You may read the entire post here.]
"Is it a VW Bug? Is it a little red Toyota truck with rusted wheel wells? Is it a sleek, black Jaguar? A yellow Jeep with fuzzy, pink dice hanging from the mirror?
"The more specific you are, the more clearly readers will see the “movie” in your head—and be drawn into your imaginary world."
In your quest to be specific, however, remember that if some is good, more is not necessarily better. If you describe every detail in the scene or setting minute, concrete detail, your story will grind to a halt and you stand a good chance of losing readers with anything less than a Herculean attention span.
In general, a few specific, evocative details, leaving plenty of room for your reader to fill in the rest, work best.
Deren blogs daily at The Laws of Making.