This is one of the 5 scrolls which appear in the bible, and it tells the story of the Jews' salvation in the time of the Persian Empire. According to the story, Ahashverosh- the king of Persia at the time- held a feast and invited the queen- Vashti. Vashti refused to come and entertain the king and his advisers. So Ahashverosh ordered to kill her, and married a second queen- Esther (also called Hadassah), the beautiful niece of a Jewish man named Mordechai.
Years later, Mordechai found out about a plot to assassinate the king, and warned him in advance thereby saving his life. The king's chief adviser, Haman, hated Mordechai, because Mordechai did not bow when he came across Haman. He knew that Mordecahai was a Jew, and convinced Ahashverosh to have all the Jews in the empire killed. When Mordechai found out about this, he turned to Esther for help, asking her to speak to the king. Esther knew that anyone who appeared before the king without being called was to be killed, and so she fasted for 3 days before turning to him, and so did all the rest of the Jews in Shushan. After 3 days, Esther wore her royal clothes and entered the king's courtyard. Ahashverosh invited her in and she invited him and Haman to a banquet. The next night she invited them both again.
Meanwhile, Haman told Zeresh- his wife- about his anger towards Mordechai. She advised him to construct a gallows for Mordechai, and so he did. In the castle, the king recalled that Mordechai had saved his life, and turned to Haman, asking him: "What should be done for someone the king wishes to reward?" Haman suggested that such a man should be taken around town on a royal horse, dressed in royal clothes, with someone calling "This is what is done to the man who the king wishes to reward". Ahashverosh then instructed Haman to do so to Mordechai.
That night, at Esther's second banquet, the king asked Esther what was troubling her. She revealed Haman's plans, and the fact that she was Jewish. The king was filled with rage and ordered that Haman should be hanged on the same gallows that he had prepared for Mordechai. That day the Jews were saved once again, and Purim has been celebrated ever since.