The Zohar is enormous, running to well over a thousand pages in translation, and its outward form is unexpected: it presents itself as a commentary on the Torah, following the weekly portions in order, in the manner of a rabbinic midrash. The mystical content is delivered inside that frame, as conversations between Rabbi Shimon bar Yochai and his companions while they walk between towns in second century Galilee.
The language is Aramaic, and it is a peculiar Aramaic. It does not match any attested spoken dialect, it draws its vocabulary from written sources, and it contains constructions that read as Hebrew translated word by word. That linguistic evidence is one of the main grounds of the authorship argument treated in the other article on this page.
Within the whole there are distinct sections that circulate almost as separate books, including the Idra Rabba, the Great Assembly, in which the companions gather to expound the configuration of the divine countenance and three of them die of the intensity of it. Those set pieces, rather than the running commentary, are what most readers encounter.
The work also spread very fast for a text of its length and difficulty. Within a century of its appearance it was being cited as ancient authority across the Mediterranean, and by the sixteenth century Christian scholars were reading it in Latin for evidence of doctrines it does not contain.