Brown sugar is common sucrose sugar but because it contains a small amount of molasses is has a brown hue.
Brown sugar is made by either refining cane sugar less or not at all leaving it with some molasses or by adding molasses back to fully refined white sugar. The latter approach insures a more consistent product because the manufacturer can control the ratio of refined sugar to molasses.
Brown sugar is distinct from white sugar because of its molasses flavor. It caramelizes better than refined sugar, making it better to use in spices, glazes and gravies.