http://www.jewfaq.org/torah.htm#Talmud
In addition to the written scriptures we have an "Oral Torah," a tradition explaining what the above scriptures mean and how to interpret them and apply the Laws. Orthodox Jews believe G-d taught the Oral Torah to Moses, and he taught it to others, down to the present day. This tradition was maintained only in oral form until about the 2d century C.E., 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 C.E.
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.
There have been additional commentaries on the Talmud by such noted Jewish scholars as Rashi and Rambam. Adin Steinsaltz recently completed a new edition of the Talmud, with his own commentary supplementing the Mishnah, Gemara, and Rashi commentaries.
The Talmud is not easy to read. It reminds me of someone else's class notes for a college lecture you never attended. 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.