A juz (plural ajza), also called a para in South Asia, is one of thirty roughly equal divisions of the written Quran. The division is made by length rather than by meaning, so a juz usually begins and ends in the middle of a surah; each is further split into two hizb and each hizb into quarters. Reciting one juz a night completes the whole Quran across a lunar month, a rhythm tied above all to Ramadan, when the entire text is recited in the nightly tarawih prayers. This portion spans Al-Baqara 2:142 through 2:252. This portion carries the change of the prayer direction (qibla) toward the Sacred Mosque at Mecca (2:144), the ordinances of fasting in Ramadan (2:183-185), rules of pilgrimage, marriage and divorce, and the narrative of Saul (Talut), Goliath and the young David that closes it.
The consonantal text these verses share across every copy of the Quran in use today goes back to a standardisation carried out under the third caliph, Uthman ibn Affan, traditionally dated to the years around 650-656 CE: a committee led by the Prophet's former scribe Zayd ibn Thabit produced an authoritative written text, working in part from manuscript sheets kept by Hafsa, one of the Prophet's widows, and Uthman then sent copies to the empire's major cities while earlier variant collections were withdrawn from circulation. The juz division used to organise recitation came only later, laid over this already-fixed consonantal text rather than shaping it, so a juz boundary never marks a textual variant, only a convenient stopping point for reciters.