Dark Brown vs. Light Brown Sugar
Brown sugar is simply granulated "white" sugar that has been combined with pure cane molasses. Molasses is a natural by-product of the sugar-making process. The difference between light and dark brown sugar is simply the amount of molasses added.
When a recipe calls for "brown sugar," it is usually referring to light brown sugar. Dark brown sugar should be used only when specified. This is important when baking recipes sensitive to moisture and density (such as cakes) because of the difference in moisture content between the two types of brown sugar. In other applications, substituting dark brown sugar over light brown will yield a deeper flavor with more caramel, much like adding molasses would do.