The Talmud is a combination of a core text, the Mishnah and a later commentary, called the gemara (literally means "addition"). There is only one Mishnah, but there are two distinct gemaras. Both gemaras were developed by many rabbis over a few centuries. One gemara was developed in Israel, near Galilee, and was redacted together in a formal collection around the year 450 CE. The other gemara developed in Babylonia. amd was redacted together in a formal collection around the year 550 CE. However, a great deal of later editorial work was done on this text for the next 250 years, so the much of the text did not reach its final, modern form until around 800 CE. The gemara is never printed by itself; it is always printed along with the Mishnah.
The Babylonian gemara and Mishnah printed together is called Talmud Bavli (the "Babylonian Talmud"). The Israeli gemara and the Mishna printed together is referred to as Talmud Yerushalmi (the "Jerusalem Talmud"; also called the "Talmud of the Land of Israel" or the "Palestinian Talmud").
The gemaras do not stick closely to the Mishnah's text; they offer a huge amount of additional material, some of which is only loosely connected to the Mishnah. They supplement it with haggadic materials and biblical expositions, and are a source for history and legend.
Talmud Bavli (often abbreviated BT) is more complete and authoritative; Talmud Yerushalami (often abbreviated JT) is fragmentary and historically of less importance. When the word "Talmud" is used without specifying which Talmud is meant, it always refers to the Babylonian Talmud.
The Babylonian Talmud is much more complete than the Jerusalem Talmud, and the redaction is much more careful and precise. Still, it is by no means complete. The gemara only exists for 37 out of the 63 tractates of the Mishna. Why did these tractates remain without gemara in Babylonian Talmud? The traditional answer is that the laws of Zeraim and Toharot (except Niddah) had no practical relevance; The agricultural laws were tied only to the land of Israel. In the diaspora these laws simply were of no use. The purity laws (except for family purity) were no longer applicable, because there was no longer a Temple and sacrificial system. One might think then that there would be no Babylonian Talmud gemara on Qodashim - but there is. This is probably because the study of the sacrificial regulations is generally thought of as being on par with actually performing sacrifices.
In the usual printed editions the Babylonian Talmud comprises the full Mishna, the 37 gemaras, and the extra-canonical (minor) tractates; This comprises folio 5,894 pages, and is much more extensive than the Jerusalem Talmud.
The Talmud is the major source of Jewish practice. One might think that the Torah would serve this role, but the Torah only lists the rules; it tells little about to follow them and how to apply them to different circumstances. Although the Talmuds were not meant to be formal legal codes (other works were created for that purpose) it is the ultimate source material, in that it is used to decide matters of Halakha (Jewish law).
Orthodox Jews study the Talmud in depth, but in fact use the Talmudic legal methodology very rarely, preferring to accept opinions in later law codes as binding. Orthodox Jews study the Talmud for its own sake; This is considered a great mitzvah. Conservative Jews also consider Halakha as binding, but do not always accept the most recent and stringent opinions in the latest law codes as absolutely binding; As such they use the Talmud in the same way that rabbis of past eras used to use it. This is theoretically still an option in the Orthodox community, but in practice is used very rarely. Reform and Reconstructionist Jews usually do not teach much Talmud in their Hebrew schools, but they do teach it in their rabbinical seminaries; The world view of liberal Judaism rejects the idea of Jewish law, and uses the Talmud as a source of inspiration and moral instruction.