Molasses produced for human consumption in the United States is made from sugar cane. The two main tasks required to make molasses from sugar cane are to separate out the sugar cane juice from the pulp, and then to extract the sugar (mostly sucrose) from the juice. What's called molasses is the syrup that remains after the sugar has been extracted from the juice.
It's not that easy to extract all of the sucrose from sugar cane juice. After a first round of processing, which involves spinning the juice in a centrifuge and heating (boiling), you can get a lot of the sucrose out, but not all of it. The syrup that remains after this first round of processing is the light molasses you see in the grocery store. It's also called "first" molasses and has the mildest taste of any molasses. Another round of processing is needed to further extract more sucrose. (The removal of sucrose from the molasses syrup is not all that significant on the nutrition side of things, but it is important to the manufacturer on the economic side because the removed sucrose can be further processed and sold as table sugar). This second round of processing further concentrates the syrup and also darkens it, resulting in the dark molasses you find in most grocery stores. Dark molasses is also called second molasses.
A third round of processing is possible, and this is the round that results in the product known as blackstrap molasses. Blackstrap molasses is the thickest form of molasses, the darkest, and the densest in terms of minerals. Three rounds of heating are the reason for the very dark color of blackstrap molasses because even though many sugars have been removed from the syrup, the sugars that do remain get caramelized from three rounds of heating.
Sometimes you'll only find blackstrap molasses in natural foods stores. Because of the superior mineral content of blackstrap molasses, I prefer this version of the product. You' find significant amounts of calcium, copper, iron, magnesium, manganese, potassium, and selenium in blackstrap molasses.