There are three main types of Molasses:-
Unsulphured
Sulphured
Blackstrap
Unsulphured Molasses are the finest quality, they are taken from the juice of sun-ripened Sugar Cane whihc is then clarified and concentrated.
Sulphured - These are made from green sugar that has not been matured enough, it is treated with sulphur fumes during the sugar extracting process. It then goes through a first boiling process - the Molasses from this first boil are the best as only a small amount of sugar has been removed. The process then goes into it's second boil which makes the Molasses a much darker colour, they are also not as sweet and are not distinctively flavoured.
Blackstrap - These are Molasses that have gone through the third boil. There main use is in the manufacturer of Cattle Food and Industrial Uses. Saying that these Molasses are extremely high in Iron and are also used in the health food industry.
Here's the link to that info:
http://www.helium.com/items/319953-blackstrap-molasses-vs-regular-molasses-understanding-the-difference