Answer:
   
   The Hebrew verb 'shanah' literally means 'to repeat [what one was
   taught] and is used to mean 'to learn'. The term 'Mishna' basically
   means the entire body of Jewish religious law that was passed down and
   developed before 200 CE, when it was finally redacted by Rabbi Yehudah
   haNasi (Judah the Prince). He is usually simply referred to as
   'Rabbi'.
   
   Prior to the time of Rabbi, all Jewish Law was transmitted orally; It
   was expressly forbidden to write and publish the Oral Law, as any
   writing would be incomplete and subject to misinterpretation and
   abuse. However, after great debate, this restriction was lifted when
   it became apparent that it was the only way to insure that the law
   could be preserved. To prevent the material from being lost, Rabbi
   took up the redaction of the Mishna. He did not do this at his own
   discretion, but rather examined the tradition all the way back to the
   Great Assembly. Some of tractates preceded him; these he merely
   supplemented.
   
   During this time period (around 200 CE) the Mishna, as such, was never
   published. Instead the main study of Jewish law was conducted in
   memorized form, except for private letters and notes.
   
   The Mishna consists of six orders (sedarim). This explains the
   traditional name for the Talmud as 'Shas'. 'Shas' is simply an
   abbreviation of shishah sedarim, six orders'. Each of the six orders
   contains between 7 and 12 tractates, called 'masekhot'. Each masekhot
   is divided into smaller units called 'mishnayot'.