Who are Chazal?

Chazal is an acronym for the Sages, may their memory be blessed. They did not write the Written Torah, because it was given to Moses our teacher on Mount Sinai directly from Hashem. The Written Torah is the Torah that is read in the synagogue on Shabbat, as well as the books of the Prophets and Writings - the acronym Tanakh. Chazal wrote the Oral Torah.

The Oral Torah in essence has always existed, and it was studied. It is all the laws learned from the Written Torah, and בעיקר the laws and halachot that are not written explicitly in the Torah but required the Sages to explain the verses to the people. When they saw, about two thousand years ago, that it was beginning to be forgotten and that not all the laws were remembered, they began to put it into writing. The initial stage is called the Mishnah, and the Sages mentioned in the Mishnah are called Tannaim.

For example, Rabbi Shimon bar Yochai is a Tanna. The one who ultimately arranged the Mishnah was Rabbi Yehudah. The next stage was to gather all the words of the Sages who offered explanations of what the Tannaim said, and after arranging all their words it was called the Gemara, and the Sages of the Gemara are called Amoraim. The hidden Torah has existed since time immemorial.