"Purim" - the holiday which commemorates the Salvation of the Jewish Community in the
Kingdom
of
Achashverosh
, which included basically the entire world, after the destruction of the
First
Temple
and before the building of the Second, from the genocidal plans of the wicked Haman. Haman wanted to kill all the Jews, children and adults, in one day, the thirteenth of Adar. However, "Pride goeth before a fall," and his devilish plan wound up on his own head, as he and his ten sons spent the holiday swinging from the gallows that Haman had prepared for Mordechai.
The primary commandment related to Purim is to hear the reading of the book of Esther. The book of Esther is commonly known as the Megillah, which means scroll. Although there are five books of Jewish scripture that are properly referred to as megillahs (Esther, Ruth, Ecclesiastes, Song of Songs, and Lamentations), this is the one people usually mean when the speak of The Megillah. It is customary to boo, hiss, stamp feet and rattle gragers (noisemakers; see illustration) whenever the name of Haman is mentioned in the service. The purpose of this custom is to "blot out the name of Haman."
We are also commanded to eat, drink and be merry. According to the Talmud, a person is required to drink until he cannot tell the difference between "cursed be Haman" and "blessed be Mordecai," though opinions differ as to exactly how drunk that is. A person certainly should not become so drunk that he might violate other commandments or get seriously ill. In addition, recovering alcoholics or others who might suffer serious harm from alcohol are exempt from this obligation.
In addition, we are commanded to send out gifts of food or drink, and to make gifts to charity. The sending of gifts of food and drink is referred to as shalach manos (lit. sending out portions). Among Ashkenazic Jews, a common treat at this time of year is hamentaschen (lit. Haman's pockets). These triangular fruit-filled cookies are supposed to represent Haman's three-cornered hat. My recipe is included below.
(mitzvah information from ou.org & jewfaq.org)