The word “Talmud” is a Hebrew word meaning “learning, instruction.” The Talmud is a central text of mainstream Judaism and consists primarily of discussions and commentary on Jewish history, law (especially its practical application to life), customs and culture. The Talmud consists of what are known as the Gemara and the Mishnah.
In addition to the inspired written Hebrew scriptures, which Christians call the Old Testament, Judaism has an "Oral Torah" which is a tradition explaining what these scriptures mean and how to interpret them and apply the laws. Orthodox Jews believe God taught this Oral Torah to Moses, and to others, down to the present day. This tradition was maintained only in oral form until about the 2nd century A.D., when the oral law was compiled and written down in a document called the Mishnah. Over the next few centuries, additional commentaries elaborating on the Mishnah were written down in Jerusalem and Babylon. These additional commentaries are known as the Gemara. The Gemara and the Mishnah together are known as the Talmud. This was completed in the 5th century A.D.
There are actually two Talmuds: the Jerusalem Talmud and the Babylonian Talmud. The Babylonian Talmud is more comprehensive, and is the one most people mean if they just say "the Talmud" without specifying which one. The Talmud is not easy to read. There are often gaps in the reasoning where it is assumed that you already know what they are talking about, and concepts are often expressed in a sort of shorthand. Biblical verses that support a teaching are often referenced by only two or three words. The Talmud preserves a variety of views on every issue and does not always clearly identify which view is the accepted one.
Christianity does not consider the Talmud to be inspired in the same sense that the 66 books of the biblical canon are “God breathed” (2 Timothy 3:16). While some of the teachings from the Talmud may be “compatible” with biblical teachings, the same can be said for many different writings from many different religions. For the Christian, the study of the Talmud can be a great way to learn more about Jewish tradition, history, and interpretation, but the Talmud is not to be considered the authoritative Word of God.