Curacao is an orange liqueur.
Curacao (pronounced "cure-a-sow", as giving medical treatment to a female pig) is named after the Laraha oranges (Citrus aurantium currassuviencis) grown on the Caribbean island of Curacao, from whose peels this liqueur is made. It has a fuller body than triple sec.
Nowadays, curacao comes in several colors, the most common of which are orange curacao and blue curacao. Red, clear, and green versions are also made.
The only real reason for the existence blue curacao is to make blue cocktails.
Note: Jerry Thomas (father of the cocktail) mispelt this liqueur as "Curacoa".