The simple carbs are broken down quickly and enter the blood stream as sugar, which if not used, is then stored as
fat. Complex
carbs/fiber are broken down slowly and keeps you feeling fuller longer.
Low carb diets advocate eliminating most fruits, veggies, milk, etc and centering on high protein, high fat foods (check out the Atkins diet). Unfortunately, by eliminating dairy, veggies, fruits, etc, you also eliminate a lot of the vitamins/minerals and fiber that's so important.