Jewish Torah is the full body of knowledge, values, practices and understandings that constitute Judaism. It was given to us at Sinai and has been expounded on and expanded over the generations.
The Torah has been transmitted through the generations by way of two main bodies:
The Written Torah is the constitutional source of all our laws and as the name implies the written source of the story of the universe, in general and the Jewish people, in particular.
The Oral Torah (which is now mostly committed to writing,) comprises a vast, dynamic library of works and traditions, covering every field of Torah intellectual endeavour, formulated (in relative significance according to the individuals,) by scholars and lay people of every generation.
It encompasses law and lore from the creation of the world to the latest technological advancements, including stories of how both the great and the infamous people through the times lived and live their lives.
The two bodies of work, together with the trials and tribulations of each living generation, are the source of Jewish belief and practice. This combination provides a foundation for Torah life that is both stable and permanent and at the same time flexible and changing.
The written segment of Torah, keeps our beliefs and injunctions fixed and unchanging so that there is a consistency and foundation.
There are models of behaviour and thought that stand through the generations as paradigms of correct and incorrect behaviour. This keeps our beliefs strong and secure, able to withstand the forces of time and external influence.
The Oral segment of Torah allows for a system of beliefs that accommodates for the changing environment never becoming redundant and being forever relevant.
The fundamentals of both sections of the Jewish Torah were given to Moshe in the 40 days and nights he spent up Har Sinai (Mount Sinai.) Hashem dictated the written content and stopped Moshe to explain with the Oral content.
Moshe learned the Oral Torah and conveyed it to the generations through Yehoshua. Until the great sage, Rebi Yehuda Hanasi, one of the last Tannaim (sages of the Tannaic - Mishna era,) saw that the Oral Torah would be forgotten if not committed to writing and he wrote the 6 sedorim (Orders) of Mishna.
The explanations of the Mishna continued to be transmitted orally until the great sages Rava and Ravina also saw this being lost if not committed to writing and they composed the Gemora.
Together these constitute the Talmud.
Once the Talmud was sealed, nothing could be added to it and any halacha (Jewish law) from then on, had to be sourced in the Talmud.