Purim is a festive Jewish holiday that celebrates the deliverance of the Jews from their enemies in the biblical Book of Esther. Purim is celebrated on the fourteenth day of the Hebrew month of Adar, which usually falls sometime in February or March. Purim is such a popular holiday that the ancient rabbis declared that it alone would continue to be celebrated after the Messiah comes (Midrash Mishlei 9). All other holidays will not be celebrated in the messianic days.
Purim is so-called because the villain of the story, Haman, cast the "pur" (the lot) against the Jews yet failed to destroy them. Reading Purim Story is a central part of the Purim celebration.
Reading the Megillah on Purim
The most important Purim custom is reading the Purim Story from the Scroll of Esther, also called the Megillah. Jews usually attend synagogue for this special reading. Whenever Haman (the villain's) name is mentioned people will boo, howl, hoot and shake noisemakers (groggers) to express their dislike of him. Hearing the Megillah reading is a commandment that applies to both women and men.
Purim Costumes and Carnivals
Unlike more serious synagogue occasions, both children and adults often attend the Megillah reading in costume. Traditionally people would dress up as characters from the Purim story, for example, as Esther or Mordechai. However, nowadays people enjoy dressing up as all manner of different characters: Harry Potter, Batman, wizards, you name it! The tradition of dressing up is based upon the way Esther concealed her Jewish identity at the beginning of the Purim Story. At the conclusion of the Megillah reading, many synagogues will put on plays (shpiels) that reenact the Purim Story and poke fun at the villain. Most synagogues also host Purim Carnivals.
Purim Food Customs
As with most Jewish holidays, food plays an important role in Purim. For instance, people are commanded to send mishloach manot to other Jews. Mishloach Manot are baskets filled with food and drink. According to Jewish law each mishloach manot must contain at least two different kinds of food that is ready to eat. Most synagogues will coordinate the sending of mishloach manot, but if you want to send these baskets on your own here is a helpful article: How to Make Mishloach Manot for Purim.
On Purim Jews are also supposed to enjoy a festive meal, called the Purim se'udah (meal), as part of their holiday celebration. Oftentimes people will serve hamantaschen, special Purim cookies, during the dessert course.
One of the most interesting commandments related to Purim has to do with drinking. According to Jewish law, adults of drinking age are supposed to get so drunk that they can't tell the difference between Mordechai (a hero in the Purim story) and Haman (the villain). Not everyone participates in this custom and recovering alcoholics and people with health problems are exempt altogether. This drinking tradition stems from the joyous nature of Purim. (Of course, it goes without saying that if you choose to participate in this custom you should drink responsibly by arranging for a safe ride after your celebrations!)
Charity on Purim
In addition to sending mishloach manot (see above), Jews are also commanded to be especially charitable during Purim. Jews will often make monetary donations to charities they support during this time or will give money to needy people.