The law was handed down to the Israelites from G-d in two forms: The Written Law and the Oral Law. The Written Law, commonly known as The Bible is split into three major segments: The Five Books (or Torah), Nevi'im (the books of the Prophets), and K'toobim (literally, writings, the Newer Books) of the Bible.
The Oral Law was given with the original Torah as a guide on how to fulfill its Mitzvot (commandments). It was first codified by Rabbi Yehuda Hanasi (Judah the Prince) after the fall of the second temple in the first century CE. This original oral law is called the Mishnah and has six books. Later Rabbis wrote a commentary on the Mishnah, called the Gemarah which was in turn commented on by other Rabbis.