I tried to answer that question as best as I could in my recent post for FineCooking.com.
Four hundred years ago, the Dutch were some of the world’s greatest traders and, not coincidentally, great distillers. They’d preserve the spices, herbs, and fruit brought home on ships in flavored liqueurs and other spirits. Curacao was one of those liqueurs, flavored with bitter orange peels from the island of the same name. At the time, the liqueur would have had a heavy, pot-distilled brandy as its base.
Then the French came along (a couple hundred years later) and invented triple sec. The “sec” meaning “dry,” or less sweetened than the Dutch liqueur. The origin of the “triple” is still up for debate, but the two leading schools of thought are “triple distillation” versus “three times as orangey”. Triple sec was also clear, whereas curacaos were dark in color.
Today, triple secs are usually still clear (made from a base of neutral spirits), whereas curacaos may start that way and be colored orange, blue, and even red. Cointreau is probably the most recognized brand of orange liqueur in the triple sec style, and Grand Marnier, despite being French, is more in line with the Dutch curacao style as it has an aged brandy base.
Nerds: Do you think that's an accurate summation?
The full post is here, and it includes a recipe for the White Lady cocktail.