What is the difference between molasses and treacle?
Here is a good article explaining the different types of molasses.

Excerpt:


  True treacle dates back to Victorian times. The pale, refined molasses
  is notably sweeter and has a much more mellow flavor than molasses.
  Nowadays, treacle is a blend of molasses and refinery syrup. It ranges
  in color from light gold to nearly black. British treacle can be
  substituted for molasses in most recipes, but much less frequently
  will molasses work as a replacement for treacle. If you do substitute
  molasses for treacle, use the lightest, unsulphured molasses you can
  find.