This festival is based on a historical story recorded in the Book of Esther. King Ahasuarus had become displeased with his first wife Vashti, after she refused to display herself at a banquet. The king hunted for a new queen, and decided on Esther, not knowing she was Jewish. Esther's uncle, Mordechai, overheard a plot to kill the king, and duly informed the king who foiled the plot, promising to reward Mordechai when he needed a favour.
Haman (whose name we usually boo or curse after), the king's advisor, had planned to kill the Jews by hanging them from the gallows he and his sons (boo) had built, on a date selected by drawing lots. Esther became aware of the plan, told the king about her faith and Haman's (boo) plan, and everything got sorted nicely, Haman and his sons (boo, hiss rasp) being hung from the gallows which they themselves built.
Purim is the most joyous festival in the Jewish calendar. When we hear the Megillat Esther (The Book of Esther) read, we add sound effects to cheer or boo the relevant characters, even in shul. We may even dress up ridiculously, and shake rattles called gregors for added effect. But the best thing about Purim is that it is a mitzvah, a good deed, to get so drunk that you can't tell the difference between Mordechai and Haman(ptooi)!
Traditionally, we eat Hamantaschen (Haman's hat!), poppy-seed filled pastry triangles.
Our Rabbis point out the coincidence in the names of Purim, the most joyous day, in the liturgy of which G-ds name is not even mentioned once, and Yom Hakippurim, the most solemn day, in which we seem to be mentioning The Holy Name every minute. This is because, they say, We should infuse each event with some of the spirit of its opposite. In the time of the Messiah, everyday shall feel like Purim.