The Sages ordained that in addition to the Torah reading, we read passages from the Prophets (the Nevi’im; the second part of Tanakh) that are related to the weekly Torah portion; one berakha is recited before the haftara and four blessings after.
This reading was instituted as a result of a decree. Once, during Second Temple era, there was a decree forbidding Jews from occupying themselves with Torah. Since they could not read from the Torah on Shabbat, the Sages ordained that a passage from the Prophets be read in the same way the Torah is read; they called up seven olim and recited Birkhot Ha-Torah. After the decree was abolished and they went back to reading the Torah every Shabbat, the Sages ordained that we continue reading passages from the Prophets. This reading is called the “haftara,” as it concludes (“maftir”) the Torah reading. They even ordained special berakhot for reading from the Prophets. Since at the time of the decree seven people were called to read from the Prophets, and each one read at least three verses, the haftara must contain at least 21 verses. However, if a section comes to a close before 21 verses are read, it is permissible to read fewer verses (SA 284:1, MB 2).
So that the haftara does not seem equal to the Torah in importance, the Sages ordained that the person reading the haftara (“the maftir”) is first called up to the Torah. This aliya is therefore called “maftir,” and its purpose is to make it clear that reading from the Prophets alone is not as important as reading from the Torah. Rather, only after reading from the Torah can one continue an read from the Prophets.
Some say that the haftara must be read from a parchment scroll of the Prophets,just like the Torah (Levush). However, many Aĥaronim write that it is permissible to read it from a printed book. Le-khatĥila, it is best to read the haftara from a book that contains that entire work of the Prophets, though be-di’avad it is also permissible to read it from a printed book in which only the verses of the haftara are printed, like they appear in the Ĥumashim nowadays (MB 284:1).
The person called up for maftir must be the sole person reading from the Prophets, and the congregation shall hear his reading. One who wishes to read along quietly may do so, as long as he does not distract neighbors from hearing the reader (MB 284:11; BHL).