Molasses is actually a by-product of the process that refines white sugar from the juice of sugarcanes. During the first stage of refining the juice is boiled to crystallize the white sugar (sucrose), leaving a thick, brown syrup called molasses. Molasses is mainly sucrose that didn't crystallize as well as other sugars and nutrients that are unable to crystallize. These nutrients include vitamin B, iron, phosphorous, and calcium. Centrifuges are commonly used to separate the molasses from the sugar crystals. Molasses can also be obtained from sugar beets but it is inferior to the sugarcane type. Brown sugar is simply white sugar mixed with molasses to make it brown, more flavorful, and softer. In Britain, molasses is mixed with refinery syrup to make treacle.
Molasses was first introduced to the Americas when Christopher Columbus brought sugarcane to the West Indies in 1493. It was a vital sweetener and trade item for the early colonists who used it in baked goods, candies like taffy, and to make rum. Molasses caused quite a problem in Boston on January 15, 1919. A huge storage tank holding millions of gallons of hot molasses cracked, flooding the sticky substance all over the city. The wave killed 21 people, injured about a hundred, and caused millions of dollars in damage. This incident was dubbed The Great Molasses Flood of 1919. Molasses remained a major sweetener in America until after World War I when the price of white sugar became much cheaper.
There are three types of molasses, light, dark, and blackstrap, which are categorized depending on how they are processed. After the first boil that removes most of the white sugar the molasses may be boiled several more times to remove additional sugar. This concentrates the molasses and its nutrients, making it darker and thicker. Light molasses has only been boiled once, making it light, thin, and very sweet. Dark molasses has been boiled twice to make it darker, thicker, and less sweet. Finally, blackstrap molasses, the lowest grade, has been boiled three times or more. It is very thick and dark and has a rather bitter taste. High quality kinds of molasses, such as Barbados and New Orleans drip, are light molasses that have been boiled only once.
Molasses is either designated as "sulfured" or "unsulfured" depending on if sulfur was used during the processing of the molasses. Sulfur dioxide gas is used both as a preservative and a bleaching agent. It is thought to remove some nutrients like vitamin A and B from the molasses. The sulfur may also give the molasses an unpleasant odor and cause an allergic reaction in some people. Unsulfured molasses (which I prefer) tends to be lighter and have a cleaner sugarcane flavor than sulfured. Most brands of molasses will indicate whether they are sulfured or not.
Molasses is used in a variety of foods. Light and dark molasses can be substituted for each other in recipes, giving either a light or deep molasses flavor to the dish. Both types are used in savory dishes like BBQ sauce and Boston baked beans. They are also used in sweet dishes like gingerbread, Indian pudding, shoo-fly pie, and molasses cookies. Molasses is acidic, so it is often combined with a base like baking soda to help baked goods rise. Additionally, light molasses is often used as syrup for pancakes and waffles while dark molasses is also an ingredient in rum and some kinds of homemade beer (check out Homebrewing 204: Alternative Sugars). Blackstrap molasses is not normally eaten but instead is used in cattle feed and for making industrial alcohol. Some people think that blackstrap is healthier than the other types of molasses, but the extra nutrients it contains are negligible.