From the Purim Wikipedia page:
Purim is a Jewish holiday that commemorates the saving of the Jewish people from Haman, who was planning to kill all the Jews. This took place in the ancient Persian Empire. The story is recorded in the Biblical Book of Esther (מגילת אסתר Megillat Ester in Hebrew).
According to the Book of Esther, Haman, royal vizier to King Ahasuerus/Achashverosh (presumed to be Artaxerxes I of Persia, "Artamkhsher"), planned to kill all the Jews in the empire, but his plans were foiled by Mordecai and his cousin and adopted daughter Esther, who had risen to become Queen of Persia. The day of deliverance became a day of feasting and rejoicing.
Based on the conclusions of the Scroll of Esther (Esther 9:22): "[...] that they should make them days of feasting and gladness, and of sending portions one to another, and gifts to the poor." Purim is therefore celebrated among Jews by:
Exchanging reciprocal gifts of food and drink known as mishloach manot
Donating charity to the poor known as mattanot la-evyonim
Eating a celebratory meal known as a se'udat Purim
Public recitation ("reading of the megillah") of the Scroll of Esther, known as kriat ha-megillah, usually in synagogue
Reciting additions, known as Al HaNissim, to the daily prayers and the grace after meals
Other customs include drinking wine or any other alcoholic beverage, wearing of masks and costumes, and public celebration.
Purim is celebrated annually according to the Hebrew calendar on the 14th day of the Hebrew month of Adar (and on Adar II in Hebrew leap years that take place every 2 to 3 years), the day following the victory of the Jews over their enemies. In cities that were protected by a surrounding wall at the time of the Biblical Joshua, Purim is instead celebrated on the 15th of the month of Adar on what is known as Shushan Purim, since fighting in the walled city of Shushan continued through the 14th day of Adar. Today, only Jerusalem and a few other cities celebrate Purim on the 15th of Adar.