(From the ReformJudiasm.org website) Purim is celebrated with a public reading—usually in the synagogue—of the Book of Esther (Megillah Esther), which tells the story of the holiday. Under the rule of King Ahashverosh, Haman, the king's prime minister, plots to exterminate all of the Jews of Persia. His plan is foiled by Queen Esther and her cousin Mordechai, who ultimately save the Jews of Persia from destruction. The reading of the megillah typically is a rowdy affair, punctuated by booing and noise-making when Haman's name is read aloud.
Purim is an unusual holiday in many respects. First, Esther is the only biblical book in which God is not mentioned. Second, Purim, like Hanukkah, traditionally is viewed as a minor festival, but elevated to a major holiday as a result of the Jewish historical experience. Over the centuries, Haman became the embodiment of every anti-Semite in every land where Jews were oppressed. The significance of Purim lies not so much in how it began, but in what it has become: a thankful and joyous affirmation of Jewish survival against all odds.