There is a mitzvah to remember what Amalek did to Klal Yisrael upon leaving Egypt. This is performed by reading “Parshas Zachor”, the portion of the Torah from Parshas Ki Setzei (Devarim 25:17) where the Torah recounts briefly what Amalek did and admonishes us to remember and not forget this episode. The Torah finishes by commanding us to wipe out any vestige of Amalek. On the Shabbos before Purim, we read parshas Zachor (Deut. 25:17-19). (Shulchan Aruch 685:1)
The consensus of the Rishonim and poskim is that the obligation to read Parshas Zachor is biblical in nature. (Tosafos Brachos 13a, Eshkol Purim 10, Chinuch 603, Beis Yosef and Shulchan Aruch 685:7)
One is obligated to hear Parshas Zachor with a minyan of men. (See Magen Avraham, Mishnah Berurah 16 and Shaar Hatzion 5 for a discussion whether this obligation is biblical or rabbinic in nature) Therefore, it is extremely important for one to go to Shul to hear Parshas Zachor.
One must read Parshas Zachor from a kosher Sefer Torah. According to many this is a biblical obligation, see Magen Avraham, Mishnah Berurah. [The Minchas Chinuch 603, however, maintains that the need to read it from a Sefer Torah is only rabbinic in nature.]
Harav Ovadia Yosef zt”l explains that if one read it from a chumash he has not fulfilled his obligation. (Yechava Daas 3:53) [It should be noted that according to the Minchas Chinuch ibid. he has fulfilled his obligation on a Biblical level. However, even the Minchas Chinuch will agree that using a Chumash will not fulfill the rabbinic obligation.]
One must take care while listening to Parashas Zachor to have in mind to fulfill the Torah obligation of remembering the actions of Amalek and the obligation to annihilate them. Similarly, the one reading from the Torah must have in mind that the entire congregation will be fulfilling their obligation by listening to his reading. (Mishnah Berurah 14. See Kovetz M’Beis Levi 15 page 17 where Harav Wosner zt”l says that the custom is not to announce publicly that everyone should have in mind to fulfill their obligation before Parshas Zachar.)
In addition, the one reciting the brachos of the Maftir (for Zachor) should have in mind to fulfill the requirement of the entire congregation to recite the blessings and all those present should have in mind to fulfill their obligation by listening to him recite the blessings. (Opinion of Chasam Sofer cited in Teshuvos V’Hanhagos 2:87)
One should ensure that he hears every word of the Parshas Zachor. Harav Tzvi Pesach Frank zt”l (Mikraei Kodesh Purim 6) maintains that if one missed even one word he has not fulfilled his obligation. In this regard Parshas Zachor has the same halachos as hearing the Megillah, which according to most opinions one has to hear every word to fulfill his obligation (see Mishnah Berurah 690:5).This also appears to be the view of Harav Chaim Kanievsky shlit”a (cited in the Sefer Yismach Yisroel Purim page 8). However, according to Harav Shlomo Zalman Auerbach zt”l (Halichos Shlomo Purim page 323) even if one missed a few words one has fulfilled his obligation, as long as he heard the main message of Parshas Zachor.
The Minchas Elazar of Munkatch zt”l writes that one is not required to read Parshas Zachor along quietly with the Chazan, however, it is preferable to do so. Harav Ovadia Yosef zt”l disagrees with this view. He writes that it is preferable to listen quietly to the reading of the chazan without reading along with him. He explains that it is much better to focus on listening to the chazzan who is reading from a kosher Sefer Torah, than to place any focus on one’s own reading from a Chumash. (Yechava Daas 3:53)
It is well known that Sefardim and Ashkenazim differ in their pronunciation of many letters. The poskim discuss whether an Ashkenazi can hear Parshas Zachor in a Sefardic shul and vice versa. Harav Ovadia Yosef zt”l (Yabia Omer 6 O.C. 11:6) writes that he would often tell Sefardic students who learned in Ashkenazi Yeshivos that for Parshas Zachor they should make sure to hear the Sefardic pronunciation. Similarly, Harav Tzvi Pesach Frank zt”l (Mikraei Kodesh page 88) would urge Ashkenazim to hear Parshas Zachor in Ashkenzic shuls and not in Sefardic shuls. Harav Shlomo Zalman Auerbach zt”l (Halichos Shlomo page 323) explains that although l’chatchila one should hear Parshas Zachor with the pronunciation based upon his family custom, however, b’dieved one fulfills his obligation under all circumstances. It is extremely common that sefardim daven in Ashkenazic shuls and vice versa and since it is difficult to leave one’s shul even for one Shabbos, one should absolutely consult with his or her rav before davening in a different location.
There is a dispute among the Poskim whether or not women are obligated to hear the reading of Parashas Zachor. It would seem that women are obligated to hear Parshas Zachor since it is a positive commandment that is not time based. However, the Sefer Ha’Chinuch maintains that women are exempt from hearing Parashat Zachor since the underlying reason for reading this portion is to remember the actions of Amalek in order to wage war against them and women do not usually take part in active combat nor are they commanded in the Mitzvah of fighting; therefore, they are not obligated to hear the reading of Parashat Zachor. Harav Chaim Kanievsky shlit”a writes that this is also the view of the Chazon Ish (Sefer Taama Dikra 23) and Harav Gavriel Zinner shlit”a (Netai Gavriel page 153) writes that this was the view of the Brisker Rav. This is also the view of Harav Shmuel Wosner zt”l (Kovetz M’Beis Levi 15 page 17). Harav David Feinstein shlit”a (Vedibarta Bam page 501) also feels that women are exempt from hearing Parshas Zachor, however, those that chose to hear the reading have performed a mitzvah.
The Minchas Chinush questions the assumption of the Chinuch by pointing to the fact that we are not permitted to speculate what the reasons are for certain mitzvos and apply special exemptions based on our own reasoning. Who is to say that the mitzvah of reading Parshas Zachor is at all related to the mitzvah of fighting Amalek? Perhaps even when Amalek is completely obliterated we will still be commanded to remember them. Many poskim, including the Minchas Chinuch, Rav Nosson Adler zt”l (Binyan Tzion 8), Minchas Elazar (2:1-5), Harav Shlomo Zalman Auerbach zt”l (Halichos Shlomo Tefila page 69 footnote 68) and Rav Ovadia Yosef zt”l (Yabia Omer 8:54), are of the opinion that women are obligated to hear Parashas Zachor. Therefore, a woman should do her best to come to shul to hear Parshas Zachor (opinion of Rav Moshe Feinstein zt”l cited in Halichas Bas Yisroel 22:1).
All agree that if a woman cannot go to shul, she should read Parshas Zachor herself from a chumash, since according to the Minchas Chinuch one can fulfill the biblical obligation by reading it in a chumash (Yabia Omer ibid.).
It is quite common for shuls to have a special reading of Parshas Zachor following shul specifically for women who could not come for davening. No blessing is recited during this reading. (Minchas Yitzchok 9:68) Harav Yosef Shalom Elyashiv zt”l (oral ruling cited in Halichas Bas Yisrael page 296) maintains that one must make sure that a minyan of men is present during this reading. It would not suffice to just have ten women present. Harav Shmuel Wosner zt”l (Kovetz M’Beis Levi 15 page 17) and others, however, opposed this entire practice of reading the Torah just for women (even if ten men are present). For practical halacha a rav should be consulted.