Answer:
   
   The Torah makes it clear that it was being transmitted side by side
   with an oral tradition. Many terms and definitions used in the written
   law are totally undefined. Many fundamental concepts such as shekhita
   (slaughtering of animals in a kosher fashion), divorce and the rights
   of the firstborn are all assumed as common knowledge by text, and are
   not elaborated. Some specific examples:
     * In describing the proper way to slaughter animals for food, the
       Torah writes "If the place which G-d your L-rd has chosen to place
       His name there will be too far from you, then you shall kill of
       your herd and of your flock which G-d Lord has given you, as I
       have commanded you." (Deut 12). However, the Torah doesn't record
       that earlier commandment anywhere.
     * When it comes to divorce -- the bible never discusses the laws
       outright, they are assumed in passing in a discussion about when
       remarriage would be allowed. (Deut 24:1-4)
     * There is a reliance on sages for interpreting the law in Exod
       18:36 and in Deut 17:8-3.
       
   Another story related to this: R' Akiva was 40 years old before he
   took an interest in Torah study. He joined a class of little children
   studying the Hebrew alphabet. On the first day, the teacher taught
   that such was an alef, and such was a beis, etc... On the second day,
   the teacher went through the alphabet backwards -- starting with tav
   and working down to aleph. R' Akiva asked the teacher, "But didn't you
   teach it the other way yesterday?" "And how do you know that that was
   the right way and not this one?"
   
   There's an alternate version, perhaps of the same story. This one is
   told about a non-Jew who came to Shammai and said that he wanted to
   convert on condition that he would accept only the Written Law.
   Shammai, realizing that the non-Jew was mocking him, chased him away.
   The non-Jew then went to Hillel with the same condition. The first
   day, Hillel taught him alef, bais, gimel, dalet. The second day, he
   began by calling the same characters tav, shin, raish, kuf. The
   non-Jew objected, "But didn't you tell me yesterday that these were
   alef, bais, gimel, dalet?" Hillel responded, "You see that even the
   names and sounds of the letters can only be understood by an oral
   teaching. How much more must the Torah itself be understood only
   through the Oral Law." The non-Jew then began studying completely and
   honestly.
   
   And an experimental proof: There were numerous movements that tried to
   follow the written Torah alone: Baithusians, Saducees, Karaites,
   etc... Each, without fail, eventually evolved its own tradition about
   how to understand the text. Pure fundamentalism about the verses,
   letting each man interpret for his/herself, has yet to provide a
   consistant structure. The Torah requires more information than it
   gives in the text alone. [Note that even Reform uses traditional
   interpretations of the verse; it is not the interpretation of the
   verse that is subject to individual choice in Reform, it is whether to
   incorporate the practice].
   
   There are a number of examples in the rest of Jewish scriptures that
   show consistancy with conclusions contained in the Oral Torah based on
   the Pentateuch. In other words, things the prophets assumed about
   Jewish law that aren't in the text:
     * Zacharia 7:2 and 8:13 refer to the Rabbinically enacted fasts to
       commemorate the fall of the first Temple.
     * Nechemia 13 notes the Rabbinic prohibition against buying or
       selling things on the Sabbath.
     * The book of Ruth only works with the Oral Torah that limits the
       prohibition of Deut 23:3 to remarrying Moabite men. Otherwise, how
       could Boaz marry Ruth -- a Moabite convert. Ruth also relies on
       Oral Torah laws on kinsman redeemers and the conversion ritual.
       
   The term "oral law" thus reflects the knowledge about how to fulfill
   the laws and regulations of Torah that was transmitted orally, from
   generation to generation. The Oral Law can be thought of as a body of
   jurisprudence and procedure that accompanies the statutes of the
   Written Law. It is believed to have been passed down from the time of
   Moses, restored after the first exile by Ezra and Nehemiah, and
   finally written down by the academies at Yavne and in the Galilee in
   the two generations following the destruction of the Second Temple in
   70 CE. It consists of specific interpretations and elaborations of the
   Written Law, and some commentary on the principles by which the
   Written Law can be expounded.
   
   There are Jews called Karaites, recognized by the state of Israel as
   100% Jewish but heretical, who reject the Oral Law, as did the
   Sadducees of the time of the Second Temple. One objection to their
   `purism' is that they have been forced by practical necessity to
   develop interpretations and methods of textual analysis of their
   own---you simply cannot have law without jurisprudence. This being the
   case, most traditional Jews accept the authority of the Oral Law that
   has come down to us as (at the very least) the closest we can come to
   Torah from Mount Sinai.