I found out what the difference was between light and dark brown sugar when I was making an Oatmeal Raisin cookie recipe. I usually only have light brown sugar available and the recipe called for dark. I looked on the internet found that all I had to do was add molasses to brown sugar. The next time I made the cookie (it is a well sought after recipe) I used dark brown sugar. Turned out that adding the molasses to the light brown sugar tasted much better.