Shabbat Zachor (“Sabbath [of] remembrance שבת זכור) 

The Shabbat immediately preceding Purim is called Shabbat Zachor. Zachor means to remember, and the Torah commands us to “remember what Amalek did to you on your way out of Egypt” (Deut. 25:17). The Sages explain that the definition of “remembering” is to speak the matter aloud at least once a year. If one accidentally missed Parshat Zachor, the obligation may be fulfilled by listening to the Torah reading on Purim itself, or to the weekly reading of Parshat Ki Tetzei(and according to many authorities, Parshat Beshalach as well). It is the widespread custom for women to attend synagogue and hear the reading.
The portion of Amalek (Deut. 25:17-19) is read, since Haman was a descendant of Agog, King of Amalek. One should be very careful to listen to all the words, since most halachic authorities consider it a Torah-level mitzvah to hear this portion once each year.

You will notice that the reader repeats the word zaicher and zecher in the last verse, since there are two different opinions as to how that word is pronounced. This stresses the importance of hearing every word correctly.
The Torah instructs Jews to “remember Amalek,” a commandment fulfilled each year by publicly reading this passage on the Shabbat before Purim, because Haman, the arch-villain of the Scroll of Esther [megillat Esther], who tries to kill the Jews of Persia, is an Amalekite. The haftarah reading is I Samuel 15:2-34, which describes Saul’s war with Amalek.