The Oral Torah passed from generation to generation and was never written down. Why? Because it was supposed to remain fluid. The principles were to stay the same, but they were to be applied to all types of new circumstances.
This worked exceptionally well as long as the central authority, the Sanhedrin, remained intact, and the chain of transmission was not interrupted. That is, teachers were able to freely pass on their wisdom to the next generation of students. But in the days after the destruction of the Temple (70 C.E.), the Sanhedrin was repeatedly uprooted and teachers had to go into hiding.
Rabbi Yehudah HaNasi (135 – 217 CE) realised that things were unlikely to improve soon. He saw that the Temple would not be rebuilt in his generation and possibly not for many generations to come. He saw the Jews fleeing the land as a result of constant persecution and impossible living conditions. He saw that the central Jewish authority was weaker than ever and might collapse altogether (which did finally happen in the 4th century C.E.).
To make sure that the chain of transmission would never be broken, he decided that the time had come to write down the Oral Torah.
This was a mammoth undertaking. Rabbi HaNasi consulted as many rabbis as possible in order to extract from them their memories. He asked them to tell him all they knew about the legal traditions they received that could be traced back to Moses at Mount Sinai. He put all those recollections together, edited them, and the end result was the Mishnah.
The word ‘mishnah’ means ‘repetition’. The Oral law was studied by repeating it and hearing it repeated; ‘mishnah’ by extension then, also means ‘learning’.
Six categories of Jewish Law
Rabbi HaNasi divided the Mishnah into six basic categories, dealing with six basic areas of Jewish law. Each of these categories is known as a Seder or ‘order’:
1. Zeraim, literally ‘seeds’, covering all agricultural rules and laws for foods as well as all blessings
2. Moed, literally, ‘holiday’, dealing with the rituals of Shabbat and other Jewish holidays
3. Nashim, literally ‘women’, examining all the issues between men and women such as marriage, divorce, etc.
4. Nezikin, literally ‘damages’, covering civil and criminal law
5. Kodshim, literally ‘holy things’, concerning laws of the Temple
6. Taharot, literally ‘pure things’, concerning laws of spiritual purity and impurity
Rabbi HaNasi finished writing the Mishnah in 219 C.E. in the town of Tzipori in the Galilee. In the side of a mountain at Beit She’arim, archeologists found a series of catacombs and also a tomb with his name on it, along with the tombs of many other great scholars of his time.
Eliezer Segal: The Mishnah
Jewish Virtual Library: Mishna/Talmud/Gemara
My Jewish Learning: Overview: the Mishnah