Treacle is a British thick, dark, sweetener very similar to molasses. Treacle has a variety of uses. You can use it to top your morning cereal, add it to warm milk to make a sweetened beverage or use it in baking.
Light treacle is referred to as golden syrup while the darker variety is called black or dark treacle. The flavor of black treacle is even more intense than black strap molasses. Black treacle is typically used when baking dark breads, cakes or puddings including the traditional dessert, Parkin. Parkin is a Yorkshire cake made with ginger, oatmeal and treacle.
If you're in the UK this is an easy to find product. In the U.S. you can look for it in specialty stores such as Williams Sonoma or Whole Foods. You can also find it online from several suppliers at Amazon.com: Treacle