Both are forms of molasses, with the syrup being a lighter molasses and treacle a very much darker form. Golden Syrup is light, buttery, and rich in flavour; Black Treacle has the distinctive taste of blackstrap molasses, which is slightly less refined and has a correspondingly higher nutritional content, so they can be substituted for one another with ease.
In Britain, and in some import shops, Tate & Lyle's Golden Syrup comes in a green and gold tin; in Wegmans, where I was thrilled to find it, it comes in a glass jar. On phoning home, I was assured that no-one in the family had ever seen it in a glass jar, so this must be something to do with selling over here - perhaps actually being able to see what's in the container is helping sell the product to Americans unfamiliar with it.
Tate & Lyle also make black treacle (in a red and gold tin), which is lovely on toast or Staffordshire/Cheshire/Derbyshire oatcakes, and is used often in making treacle toffee (where I come from, it's a special Bonfire Night treat).
Both products are also marketed by other firms in the UK, and can be found all over the country, but the distinctive Tate & Lyle's tins are the most familiar kinds.
You can read all about treacles here.