Haman (also known as Haman the Agagite) was the main antagonist of the Book of Esther. He was King Xerxes' former second-in-command and prime minister who had a deep hatred for the Jews and Mordecai, because Mordecai won't bow to him.
In the story, Haman arrives riding on a camel, being introduced as King Xerxes' right-hand man (but what the people didn't know was that he was really a bad guy). Then Haman orders the Jewish people to get on their knees and bow to him, except one Jew named Mordecai, who knows Haman as an Amalekite (whose kind doesn't honor God, and were ancient enemies of the Isralites), will not.
This angers Haman and he tells King Xerxes that the Jews wouldn't obey the laws and that he must make a new order saying to kill all the Jews; especially Mordecai, whom Haman plans to hang.
Finally, when Queen Esther explains to Xerxes that she's part of Haman's plan, a Jew, and Mordecai's cousin, the angered and betrayed King yells at Haman that he lied to him and he then orders the guards to hang Haman on the gallows that he'd prepared for Mordecai. Afterwards, after Haman's death, the King hires Mordecai to be his new helper and the Jews don't get killed off after all.