Orange Curaçao traditionally is made from the peels of the bitter oranges from the island Curaçao. Orange Curaçao can be colored orange, blue or left clear (white). All of these taste the same, but provide a different visual effect to the resultant drink.
Orange Curaçao is similar to Triple Sec in that it is a sweet orange flavored liqueur, but where Triple Sec is normally made with a neutral grain spirit base, Orange Curaçao is usually made with a brandy base. In a pinch however you can substitute one for the other.
Grand Marnier is a "top shelf" Orange Curaçao.