Molasses is a byproduct of the sugar-making process, and is made with both sugar cane and sugar beets. Specifically, molasses is the concentrated cane syrup or beet syrup leftover when sugar crystals are extracted.
The most widely available molasses is sugar cane molasses because it is often used in baking. It comes in three varieties: light, dark and blackstrap. When sugar cane is being processed into sugar, the juice from crushed or pressed sugar cane is boiled to prompt the crystallization process. The liquid resulting from the first cooking of the sugar cane syrup is light molasses. It has a relative high sugar content and a fairly mild flavor. Dark molasses is the liquid resulting from the second boiling of the sugar can juice. It is less sweet, quite a bit thicker and darker in color than light molasses. Dark molasses is sometimes called “robust molasses,” as well. Blackstrap molasses is the darkest, thickest and least sweet of the types of molasses and is the result of the third and final boiling of the sugar cane juice.
Molasses can also be sulphured or unsulphured. Sulphur was added to sugar cane juice during production to act as a preservative, to kill unwanted bacteria and even to help whiten the resulting sugar crystals. Recipe-wise, there should be no difference if you use either, but the vast majority of molasses produced these days is the unsulphured variety, so the question is unlikely to arise at all.
Unless it is labeled differently, the average jar of molasses in the supermarket is probably unsulphured light molasses. It is also the type of molasses that most recipes are developed for. If a recipe doesn’t specifically call for dark or blackstrap, you’ll want to experiment with them carefully because they can completely change the flavor of a gingerbread or molasses cookie.