There are two types of treacle.
The kind of treacle that is in treacle tart, which is probably what you're asking about, is like a much thicker version of Golden Syrup (you have that, right?). Treacle tart is like jam tart, but one large one that's cut into pieces for everyone eating it rather than individual tarts. Also it's served hot and with custard poured over the top.
There also exists something called black treacle that is much more strongly flavoured. This is pretty much the same thing as molasses, afaik.
Just ask me all your sugar-related questions, and I'll be pleased to answer