Purim celebrates the salvation from the threat of extermination during the second exile of the Jewish people in Persia/Medea around 350 BCE. The great simcha (happiness) of the occasion and the Hashgacha Pratit(Divine Providence) experienced by the Jews made the Sages of that time declare Purim a holiday for all Jews, for all time. They ordained four mitzvot (commandments) exclusive to Purim.

Since the holiday is not ordained in the Torah, it is not a Biblical Mo'ed, or festival, and as such, there are no prohibitions on work during the day. There are, however, certain mitzvot that were established by our Rabbis, and which help us to get into the spirit of the day!

On the night of Purim, we come together to hear the Megilla reading. Then the mitzvah of Matanot L'Evyonim - Gifts to the Poor - unifies the poor and the wealthy. Mishloach Manot - Sending Portions - strengthen the ties between us and our friends. Then, perhaps in order to further overcome the barriers separating us, we drink. The drunkenness of the Purim meal, should rid us of our inhibitions, not to bring us to wildness, but to allow us to show our inner and deeper love for one another. By drinking (in moderation) we blur the boundaries separating us to feel even more oneness.

Al HaNissim
In addition, there is an extra paragraph (Al HaNissim) added to the Shmoneh Esreh prayers of Maariv, Shacharit and Mincha, and to the Grace (Birchat Hamazon) after eating a meal. The prayer Al HaNissim is similar in substance and content to the Al HaNissim read on Chanukah, a praise and thanks to God for the miracles that He performed for the Jewish nation at the time of Purim.

Shushan Purim
Jews who live in cities that were walled during the times of Yehoshua celebrate Shushan Purim (the day after Purim) instead of Purim. Today, the only city that is definitely in that category is Jerusalem.