Golden Syrup is a British thing; in the US, substitute golden syrup for corn syrup. Golden syrup is a pale yellow, thick but runny syrup. It is really sweet, but has a very clean taste - it's about 40% sweeter than normal sugar.
Golden syrup is used for biscuits and cakes, plus for the treacle in traditional British puddings like treacle sponge. If adding to your cakes, only a spoonful or two will give you that deep, toffee taste, so don't overdo it.