There are four commandments to be fulfilled on Purim. These are in addition to the custom of disguising ourselves in costumes in commemoration of how G-d concealed himself when orchestrating the miracle of Purim (G-d’s name does not appear in Megillat Esther). The four commandments are:
Reading the Megillah (Kriyat Megillah)
On Purim, Jews gather to read the story of the holiday as it is recorded in the Book of Esther. It is customary to make noise whenever the name of Haman is read in order to “blot out” his name.
Giving Charity (Matanot L’evyonim)
On Purim, Jews are required to give gifts to the poor in order to commemorate the charity G-d gave the Jews by overturning Haman’s evil decree. The requirement is to give at least one gift to two different needy people, at least two gifts in total.
The Festive Meal (Seuda)
In the afternoon of Purim we are instructed to partake in a festive meal. According to the Vilna Gaon this feast is commemorative of the feast Esther had with the King and Haman where she asked the King to save the Jews.
Sending Gifts to one Another (Mishloach Manot)
We are commanded to give one another food gifts on Purim. The commandment is to send one gift of two portions of ready-made food to another. This commandment is commemorative of the brotherly love that awoke amongst the Jews after the story of Purim occurred. The most common food to be given in these gifts is Hamentaschen, triangular pastry pockets with various fillings.
On Purim, there are also special prayers recited. Al Hanissim is added to the Amidah prayer and Birkat HaMazon. The Megillah is read twice, once at night and again in the morning.