Light vs Dark Brown Sugar
They have a different ratio of white sugar to molasses.

Therefore, dark brown sugar is more hygroscopic, and will have a deeper molasses flavor (and color, obviously)  They're pretty similar, and you can usually get away with replacing one with the other, but if you want subtle flavors coming through, dark brown sugar might mask it.

I've seen recommendations that call for substituting light brown sugar with a 50/50 mix of white sugar and dark brown; I don't know how messy (or if even possible) to add molasses to light brwn to get dark brown sugar.