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.

Originally the Oral Law was never 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. 

The Oral Law consists of three components:

1. Halachah L'Moshe M'Sinai:

When Moses went 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.

G-d told Moses (Exodus 24:12) "I will give you the Torah and the commandments." Why did G-d add the word "commandments?" Aren't all the commandments included in the Torah? This verse is a clear inference to the existence of the Oral Torah.

It is obvious that a Halachah L'Moshe M'Sinai, which was spoken by G-d Himself, carries the same weight as a commandment which is written by G-d in the Torah.

2. Shlosh Esreh Middot ShehaTorah Nidreshet Bahem:

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 Exodus² 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.³ 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.

It is clear that anything which is extrapolated from the text of Torah through one of the thirteen methods, is merely clarifying the intention of the Torah itself, and can therefore be viewed as the actual word of G-d.

3. Gezayrot:

The Torah⁵ granted the rabbis the authority to protect the word of the Torah through making "Gezayrot" (decrees).

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

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

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.⁶ Since that time, different communities have assumed upon themselves various stringencies, but there aren't any more universally accepted gezayrot.