Answer:
   
   The traditional view is the the Written Law was given to Moses at
   Sinai, and has remained unchanged since that time. At the same time,
   according to the traditional view, the Oral Law was dictated but not
   written down, in order to provide clarifications of Torah. To some
   extent, this is necessarily the case; the Written Torah mentions some
   core laws (e.g., the identities of kosher and non-kosher species,
   shechita [slaughtering], the kinds of activities prohibited on
   Shabbat, how Yom Kippur is observed, how the shofar is blown, what
   t'fillin [phylacteries] are, what is a sukkah, marriage and divorce)
   only briefly, without any of the requisite details. In many such
   instances, the Oral Torah has special status, and is referred to as
   "halakha l'Moshe mi'Sinai" (literally, Law to Moses at Sinai), and has
   the same immutable status as the Written Torah itself. Another factor
   "forcing" the recognition of the Oral Torah was the need for the basic
   halakhic principles of the Written Torah to extend and adapt (within
   limits) to societal changes; cultural and social changes demanded
   halakhic decisions, and these halakhic decisions had to be transmitted
   across generations. Deut 17:8-9 tells the people to "go the the judge
   who shall be in those days;" the rabbinic tradition thus explicitly
   commands adherence to the Oral Torah and to rabbinic authority.
   
   We do not know much of the early history of the Oral Torah, but much
   of it (e.g., the basic structure of the Amidah liturgy, and the basic
   principles of halakhic exegesis) is ascribed to the Men of the Great
   Assembly (539-332 BCE, the era of the Second Temple and Persian rule).
   Subsequent development of the Oral Law took place in the era of the
   Zugot ("pairs" of scholars who served as spiritual and intellectual
   leaders of the Jewish community under political domination of the
   Greeks and Hasmoneans; it was in that period that the Sadducees, who
   substantially rejected the authority of the Oral Torah, arose. But the
   varieties of modern Judaism derive from the Talmud, in which the
   essential principles of rabbinic Judaism were more fully discussed and
   developed. If the Oral Torah was indeed given to the Jews at Sinai at
   the same time as the Written Torah, how does one explain the talmudic
   disputes? There are at least three possibilities, and they are not
   mutually exclusive. Perhaps the Oral Torah was transmitted
   inaccurately, and the task of the rabbis was to reconstruct it.
   Alternatively, the halakhic principles of the Oral Torah were used by
   the rabbis to derive new laws, and to apply old laws to novel
   situations. The third possibility is that the Oral Law gave the rabbis
   the right (perhaps the responsibility) to legislate.
   
   Non-traditional movements have different positions on the origin. Some
   hold with the "documentary theory", which has four authors. Some hold
   with divine inspiration. Others believe in divine inspiration, written
   in the language and context of its time. However, all agree that the
   Written and Oral Torah contain eternal truths that apply as well today
   as when the documents were committed to parchment, and that study of
   both is critical.