How did the rabbis build the Talmud, and why is it a book of arguments?

The Talmud was built in layers, and the layer that matters most for its character is made of argument. It is a central text of Rabbinic Judaism and a primary source of Jewish law and theology, and what makes it unusual is that it records the teachings, opinions and disagreements of thousands of rabbis rather than only the conclusions.

It grew in two layers. First came the Mishnah, the first written collection of the Jewish oral traditions, known as the Oral Torah. According to the Talmud, it had not been permitted to write such teachings down, so scholarship had been predominantly oral. According to tradition, Judah ha-Nasi compiled the Mishnah, fearing that the oral traditions of the Second Temple period would be forgotten. Its statements are terse: brief opinions of rabbis debating a subject, or an unattributed ruling.

Then came the Gemara, a commentary on the Mishnah that arose from rabbis analysing, debating and discussing it, a process called shakla v'tarya, "taking and throwing". A discussion can be framed as an exchange between a questioner and an answerer, often anonymous. It also looks for the biblical basis of each law and the logic that links the two. Mishnah and Gemara together are the Talmud.

There are two Talmuds, the Jerusalem Talmud and the much longer Babylonian Talmud, with about 2.5 million words. When people say "the Talmud", they usually mean the Babylonian one.