Haman was the name of the vizier (i.e., the prime minister) of the Persian king Ahasuerus. He is called an "Agagite," which seems to denote that he was descended from the royal family of the Amalekites, the bitterest enemies of the Jews, as Agag was one of the titles of the Amalekite kings. He or his parents were brought to Persia as captives taken in war. The book of Esther records that he convinced Xerxes, the Persian king, to issue a decree calling for the destruction of Jews. The Jews were saved due to the heroism of Esther, a Jewish wife of Xerxes, and Haman was hanged on the gallows which he had erected for Mordecai.
This story is recounted in detail during the Jewish festival of Purim.