The Columbian Issue, sometimes simply called the Columbians, is a set of 16 highly prized stamps issued by the United States to commemorate the World Columbian Exposition, held in Chicago in 1893. They were the first commemorative stamps issued by the United States, and they depict various events during the life of Christopher Columbus.
For detailed information on the Columbian issues, visit: http://en.wikipedia.org/wiki/Postage_stamps_and_postal_history_of_the_United_States.