Brown sugar is white sugar that has had a small amount of molasses added to it. The molasses gives it a richer, deeper flavor than white sugar and also makes the sugar very moist. Dark brown sugar has a very strong molasses flavor, while light brown sugar is a little drier and has a much milder flavor. The two most common brown sugars are light brown and dark brown. Many grocery stores also stock golden brown sugar, which falls somewhere in between light and dark. Muscovado, which is a very dark and strongly flavored brown sugar, is also available in many grocery stores.
Although there are quite a few types of brown sugar out there, not many recipes specify what type of brown sugar they call for. Recipes don’t usually specify because the different types of brown sugars are interchangeable and will perform the same way in just about every cookie, cake, bread or other recipe that they’re included in. When recipes do make a recommendation for dark brown over light brown sugar, it’s not because of the way that the sugars function, but because of the flavors that they impart in a recipe. A darker brown sugar brings that slightly bitter molasses note to gingerbread and can add depth of flavor to chocolate cake. A lighter brown sugar is a better choice for butterscotch pudding or caramel corn, where you might want a subtler flavor in the finished product.
Since light brown and dark brown sugar are generally interchangeable, it is worth taking a chance and playing around with their flavors, seeing what they add to different recipes (such as chocolate chip cookie) and what type of sugar produces your favorite result.