Purim celebrates the salvation of the Jews of Persia from certain destruction. King Achashverosh's advisor Haman was offended when Mordechai, a Jew, would not bow to him, so he drew lots (purim in Hebrew) to determine a date upon which all of Persia's Jews would be killed: the 14th of Adar. Mordechai's relative Esther entered and won a contest to become Achashverosh's new queen after his first wife Vashti was thrown out of the palace, so she was able to use her influence to convince Achashverosh to condemn Haman and his plot.
Purim is a festive and joyous holiday. Many people dress up in costume to attend Purim carnivals, and when the megillah (scroll) is read aloud, noisemakers called groggers are shaken every time Haman's name appears. Other customs include enjoying a large meal, giving gifts to the poor, and sending ready-to-eat food (mishloach manot) to friends, often including hamantaschen ("Haman's pockets," "Haman's hat," or oznei Haman [Haman's ears] in Hebrew; triangle-shaped pastries filled with poppy seeds, jam, or even chocolate/Nutella).