Triple sec, originally Curaçao triple sec, is a variety of Curaçao liqueur, an orange-flavoured liqueur made from the dried peels of bitter and sweet orange.
Triple sec may be consumed neat as a digestif or on the rocks, but is more likely to be used as an ingredient in a variety of cocktails such as the Margarita, White Lady, Long Island Iced Tea, and Cosmopolitan.
A common question is hey, what's the difference between and triple sec and orange curaçao? Camper English of Alcademics has a good answer:
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.