The Torah has two parts: The "Torah Shebichtav" (Written Law), which is composed of the twenty-four books of the Tanach, and the "Torah Sheba'al Peh" (Oral Law).

G‑d told Moses1 that he will give him "the Torah and the commandments." Why did G‑d add the word "commandments?" Are there any commandments which are not included in the Torah? This verse (amongst others) is a clear inference to the existence of the Oral Torah.

The Oral Torah was transmitted from father to son and from teacher to discipleOriginally the Oral Law was not transcribed. Instead it was transmitted from father to son and from teacher to disciple (thus the name "Oral" Law). Approximately 1800 years ago, Rabbi Judah the Prince concluded that because of all the travails of Exile, the Oral Law would be forgotten if it would not be recorded on paper. He, therefore, assembled the scholars of his generation and compiled the Mishnah, a (shorthanded) collection of all the oral teachings that preceded him. Since then, the Oral Law has ceased to be "oral" and as time passed more and more of the previously oral tradition was recorded.

1. Laws Given to Moses at Sinai

When Moses went up to heaven to receive the Torah, G‑d gave him the Written Torah together with many instructions. These instructions are called "Halachah L'Moshe M'Sinai" (the Law that was given to Moses on Sinai). Maimonides writes that it is impossible for there to be an argument or disagreement concerning a Halachah L'Moshe M'Sinai, for the Jews who heard the instructions from Moses implemented them into their daily lives and passed it on to their children, who passed it on to their children, etc.

Some examples of Halachah L'Moshe M'Sinai are: tefillin straps must be black, a sukkah must have at least two and a half walls, and all the different Halachic measurements and sizes.

2. The Thirteen Principles of Torah Exegesis

When G‑d gave the Written Law to Moses he also instructed him how one is to study and understand the Torah. Every word and letter in the Torah is exact, and many laws can be extrapolated from an extra (or missing) word or letter, or a particular sequence which the Torah chooses to use. The thirteen principles which are the keys to uncovering the secrets of the Torah are called the "Shlosh Esreh Middot ShehaTorah Nidreshet Bahem."

For instance: One of the rules is: "Anything that was included in a general statement, but was removed from the general statement in order to teach something, was not removed to teach only about itself, but to apply its teaching to the entire generality." An example for the usage of this rule is: In Exodus 35:3 the Torah says "You shall not light fire in any of your dwellings on the Shabbat day." Now, kindling a fire was already included in the general statement that prohibits work on Shabbat (Exodus 20:10). It was removed from the general rule and stated independently in this verse to teach us that it is a distinct form of work and, as such, carries a distinct penalty. Moreover, this lesson applies to each of the 39 categories of work included in the general statement. Thus, there isn't a broad category called "work," rather each type of work is to be viewed as distinct. Therefore, if someone should do several kinds of work while unaware that they are forbidden on Shabbat, he must bring a separate sin-offering to atone for each type of work that he did.

A full list of the thirteen principles can be found in the prayer-book.2

3. Edicts

The rabbis constantly added gezayrot according to the needs of their timesThe Torah3 authorizes the rabbis to protect the word of the Torah through making "Gezayrot" (edicts).

For example: The Torah prohibition of eating or possessing chametz (leavened products) on Passover begins at midday of the fourteenth day of Nissan. Our sages added two hours to this prohibition, for they feared that on a cloudy day people would err and eat chametz after noon.

Just like the Congress is constantly enacting new laws and regulations, for the old laws are not always adequate for modern trends and tendencies, so too, the rabbis constantly added gezayrot according to the needs of their times.

Although the Torah commands us to follow these gezayrot, there are distinctions between a rabbinic decree and a Torah law. One of the distinctions is that when there is a doubt concerning a Torah law one must be stringent, whereas if there is a doubt in a rabbinic decree one may be lenient. [In case of an actual dilemma, always make sure to ask a rabbi what to do.]

Until the end of the Talmudic Era (approx. 1500 years ago) there was a central rabbinic authority which issued gezayrot which were accepted by all the Jews.4 Since that time, different communities have assumed upon themselves various stringencies, but rarely are there universally accepted gezayrot.