One is required to read the Megillah both at night and during the day. The obligation at night can be fulfilled from dark until amud ha-shachar, the crack of dawn, and the obligation to read by day can be fulfilled from sunrise until sunset.
The mitzvah of reading the Megillah applies to both men and women. Optimally, the reading should be done in the presence of a minyan (a quorum of ten men), in the synagogue. Even if one has a minyan in his home, it is still preferable to go to the synagogue for the Megillah reading since, as the verse says (Proverbs, 14:28):"The King's Glory is [manifested] in the multitude.” Since one of the reasons that we read the Megillah is to publicize the miracles of Purim, this is best accomplished when it is read publicly in the synagogue.
The reading of the Megillah takes precedence over the performance of all positive Torah precepts, even the study of Torah is suspended for the Megillah reading. The only mitzvah which we do not suspend for the reading of the Megillah is the mitzvah of providing burial for a person found dead, who has no one else to bury him.
One can fulfill the obligation of reading the Megillah by hearing it being read, for this is considered reading it oneself. The person reading, however, must himself be obligated in this mitzvah [e.g., an adult rather than a minor]. If one is fulfilling the obligation by listening to someone else's reading, he must be careful to hear every word, for if he does not hear the entire Megillah, he will not have fulfilled the mitzvah.
It is proper that all those listening to the reading should have a Megillah written on parchment in front of them and silently read along. In this manner, one can be certain that even if he failed to hear a word being read, he will have read it by himself and will have thus fulfilled the mitzvah. If he does not have a Megillah written on parchment, he should read along from a printed Megillah.
The established custom is to spread the Megillah out on the reader's table, rather than to roll it as one does with a Torah scroll. The sheets of parchment on which the Megillah is written are folded under each other so that the sheets do not hang over the table. The basis for this custom is the verse in Esther (9:31) which refers to the Megillah as a letter: To fulfill this Purim letter.
Just as a letter is held completely open when being read, so too should the Megillah be completely open when it is read. Moreover, the unusual manner in which we hold the scroll serves to publicize the miracle of Purim more widely.
Traditionally, the reader pauses, allowing the congregation to also recite the text of the four verses in the Megillah which speak of Israel's redemption: “There was a Jewish man in Shushan (Esther 2:5); And Mordechai went from before the king in royal clothing (ibid. 8:15); The Jews had illumination (ibid. 8:16); and the last verse of the Megillah, For Mordechai was deputy to the king (ibid. 10:3).” The reader then proceeds to repeat these verses since those who have their obligation fulfilled by listening to the Megillah, rather than reading it themselves must hear every word. The purpose of this custom is to intensify the joy and to keep the children from falling asleep, so that the story of the great miracle performed on Israel's behalf during the time of Mordechai and Esther will enter their hearts.
It is also customary to read the verse: “That night the sleep of the king was disturbed” (Ibid. 6:1), using a different and louder melody for the cantillation because this verse marks the point where Israel's salvation began.
The names of Haman's ten sons, the phrase five hundred men which precedes them, and the word ten which follows (Ibid. 9:6-10) are traditionally read in one breath, thereby indicating that they were all killed at one time. The five hundred men mentioned indicates that they were all followers of Haman's sons who served as their commanding officers. If the reader failed to read these passages in one breath, one has nevertheless fulfilled the mitzvah of the Megillah reading.
NOTE: In the translated Megillah text, available on the site, the passages that need to be recited in public are in bold.