Esther is described in the Book of Esther as a Jewish queen of the Persian king Ahasuerus. In the narrative, Ahasuerus seeks a new wife after his queen, Vashti, refuses to obey him, and Esther is chosen for her beauty. The king's chief advisor, Haman, is offended by Esther's cousin and guardian, Mordecai, and gets permission from the king to have all the Jews in the kingdom killed. Esther foils the plan, and wins permission from the king for the Jews to kill their enemies, and they do so.
Her story is the traditional basis for Purim, which is celebrated on the date given in the story for when Haman's order was to go into effect, which is the same day that Jews kill their enemies after the plan is reversed.