Photo: Fairywren, Flickr
Both the light and dark varieties of brown sugar are a mixture of plain, white granulated sugar and molasses. In fact, you can even make your own.
The only difference? Dark brown sugar contains more molasses than its light counterpart.
Light brown sugar is subtler and the more commonly used variety -- best for baking, making butterscotch and creating glazes. Dark brown sugar and its stronger molasses flavor is more suited for full-bodied recipes like gingerbread and baked beans.
However, the two types are not so different that they can not be used interchangeably. Just note the light offers up a more delicate, caramel flavor while dark will add a touch more robustness and molasses flavor.