Just a short background for anyone who doesn't know.
When Moses got the Torah on Mt. Sinai, he also recieved the oral code - the "Torah Sh' Ba'al Peh", which was passed down from him on to Joshua, and on to the sages of Israel.
For many centuries, there existed no ambiguity as to the meaning of the oral code. However with time, there appeared different versions of the code, due to different interpertations of the words.
When the Romans started to oppress the Jews, Rabbi Judah HaNasi decided that the oral code must be written down, or else it may be completely forgotten.The written version of the oral code is today's Mishna.
The Mishna is devided into 6 volumes (Seder in Hebrew):