The oral law was eventually redacted into a written form and is known as the Mishna and Gemara that we have at our disposal today. The first editors of the latter, Rabina and Rav Ashi are known in the Talmud as being “the end of instruction”. That is, upto their generation the learning in the academies was always oral in nature. Only in the subsequent generations did it become written in nature.
One must stress at this point that this is not just a historical point but one that has tremendous halachic ramifications as well. The medieval commentators are able to argue with an opinion which was formulated after the period of the Gemara but can not disagree with a section which is part of the oral tradition. Therefore any section which comes after the period of Rabina and Rav Ashi simply can be considered as commentary and not binding on the student. Only those parts written in the generation of, or in previous generations, are binding on the student. Thereafter, we find a tremendous increase in argumentation of commentaries. Rabbi Yosef Karo found in his day that the amount of commentaries had become so great that he felt compelled to make decisions as to what should be the Halacha. As a result he wrote his “Shulchan Aruch”.
A fascinating question that has bothered the halachic world is why the Babylonian Talmud is so different in nature than its Yershhalmi equivelent.The former is a far more open book, easier to read and written in a easier style. The reason for this is historical but also has halachic importance as well. It is clear that the students of Rabina and Rav Ashi continued the work of their Rabbis and the Talmud is made up of many parts that were only redacted generations after their death.The final editing of the Gemara together with key phases and conjunctions are attributed to this group of scholars who are historically known as the Savoraim. It is their key words and grammatical additions that make the Babylonian Talmud the easier of the 2 Talmuds to learn.
As a result there are a number of occasions where the Rambam decides against the apparent simple understanding of a certain section of the Gemara.The reason according to some Halachic scholars (Sde Hemed and others) is that the Rambam was able to decipher between those sections written in Rabina and Rav Ashi´s time which are binding and those written after their time which is only classified as commentary.There are many occasions where the medieval commentaries stipulate that a certain section of Gemara is a later addition and therefore not binding. However it appears that certain commentators see the entire Talmud, as we have today, as being binding and do not agree with the view that one can divide the material up into different historical layers. There are today scholars who believe that they possess a method by which one can decipher the different segments of the Gemara but their opinion has not been verified by the leading authorities of our generation.
As we have mentioned, the Gemara represents the end of “instruction” and thereafter we live in the period of commentary. However, the Messora continues in the form of minhag and Psuk Halacha. The great school of Brisk for example has its traditions of how certain questions are answered. When one reads the opinion of Rav Soloveitchik, for example, he is in fact often reading classical Brisker traditions. The leading authorities of our day often continue the halachic traditions and methods set by their own teachers.