Oral Torah

When Moses received all of the laws that would define the Jewish tradition, he also received the explaination of these laws. The laws were contained in the Pentatuch or Torah. These were written down on scrolls. But the explaination was not allowed to be written down. Jews were obligated to speak the explaination and tell it to students, children, and fellow adults. When Israel was eventually destroyed by Rome in 70 CE, Jews who memorized the explaination were being killed. So the explaination wouldn't die as well, a Chief Rabbi, 200 years later had it written down. The Oral Law became the Talmud.