Molasses and treacle aren't exactly the darlings of haute cuisine: other sweeteners such as maple syrup or honey made from exotic flowers are much more popular and fashionable. Somewhat old-fashioned, molasses and treacle are usually used in rustic, homey dishes: the type you might see in the rural regions of the southern United States or in boarding-school dinners in Britain.
What are they? They're both by-products of the sugar-making process. Although they're made in slightly different ways, the tastes are similar and
they can be used interchangeably.
Types: there's light molasses, which is similar to golden syrup, and dark molasses, which is similar to dark (or black) treacle. Blackstrap molasses, another type, doesn't seem to have a British equivalent. Recipes will call for a specific variety (dark or light). Don't try to substitute one for another because it will ruin the flavour balance.
What's the difference? Light molasses comes from the first boiling of the sugar extraction process of cane or beets. It's the sweetest of the different types and also the lightest in colour. Dark molasses/treacle comes from the second boiling of the sugar and has a darker colour and a deeper, more complex flavour. Blackstrap molasses is made from the third boiling of the syrup and is thick, dark and slightly bitter.
Sulphured or unsulphured? Sulphur fumes are sometimes used to process molasses. This isn't common, though, and sulphured molasses is considered to be of inferior quality.