The Sages instituted that in addition to the Torah reading, there should also be a public reading from the Prophets on a topic related either to the Torah reading or to the time of year. One berakha is made before this reading, and four afterward. This reading is called the haftara, meaning “conclusion,” for with it the Torah reading concludes.
The haftara was instituted following an imperial decree that Jews may not read from the Torah, on penalty of death. However, since the decree did not apply to reading from the Prophets, the Sages of that time instituted that congregations read from the Prophets instead of the Torah. They further instituted that the reading from the Prophets be modeled on the Torah reading, with berakhot before and after, and with seven aliyot. After the decree was rescinded and Torah reading resumed, the Sages ordained that congregations continue reading from the Prophets each week, but now special berakhot were established for it. Since while the decree was in place they had had seven people reading from the Prophets, each reading a minimum of three verses, they instituted that the maftir (the person who reads the haftara) should read a minimum of 21 verses. However, if the selected passage from the Prophets is shorter than 21 verses, we conclude the reading when the subject is finished, even if it is fewer than 21 verses (SA 284:1; MB ad loc. 2).
In order to demonstrate that the haftara is not as important as the Torah reading, it was ordained that the maftir should read a few verses from the Torah scroll first, and only then read from the Prophets. Strictly speaking, the seventh oleh may read the haftara, but the custom is to follow the opinion that the maftir should not be one of the seven olim. Therefore, after the seven olim finish reading the parsha, Half-Kaddish is recited, thus concluding the Torah reading. Then the maftir is called up to read a few verses from the Torah scroll, and he then goes on to read the haftara from the Prophets.
Some maintain that just as the Torah is read from a parchment scroll, so too the haftara must be read from a parchment scroll (Levush). However, many Aĥaronim maintain that the haftara may be read from a printed volume. It is better to read it from a volume that has the entire text of the biblical book from which the haftara is excerpted, but if this is not available one may read from a book that has only the verses of the haftara, as is the case with many of our printed ĥumashim (MA; Eliya Rabba; MB 284:1).
Some have a custom that the entire congregation reads the haftara together; however, it is more correct that the person who is called up for maftir or the ĥazan reads it alone, while the congregation listens to his reading. One who wishes to read along in an undertone may do so as long as he does not disturb the person sitting next to him (MB 284:11; BHL ad loc.).