The Pentateuch consists of the first five books of the Bible: Genesis, Exodus, Leviticus, Numbers, and Deuteronomy.  It is derived from the Greek "pentateuchos" meaning "five containers" or "five-volumed."  It enjoys particular prestige among Jews as the "Law" or "Torah," the concrete expression of God's will.  It also has the benefit of being containing five books that have NEVER had their canonicity called into question (by either Jews, Catholics, or Protestants).  The division of the material into five sections is supported by both the Septuagint, a 3rd century B.C. translation of the Hebrew Old Testament into Greek, as well as the Samaritan Pentateuch, which dates from even earlier. 

As for material, it contains: a body of legal doctrine, the formation of the People of God (Abraham and the patriarchs), Moses and the oppressed Hebrews in Egypt, the birth of Israel in the Sinai covenant, the journey to the promised land, and the "discourses" of Moses.  The time frame is from creation to the end of the Mosaic era. 

Many scholars divide/identify the material as coming from four major historical traditions/sources: Yahwist (J), Elohist (E), Priestly (P), and Deuteronomic (D).  This theory is called the "Documentary Hypothesis."  However, this is a simplification, and one that many scholars are now rejecting, due to the simple fact that all four of these "divisions" draw upon even older material from different sources (not just from four major sources), and that it ignores what we know of literary traditions of the time (for example, how stories would often be told from two or more points of view, even by the same author).  However, other scholars feel that modifying the initial hypothesis, not rejecting it outright, is the proper course of action.  By looking at the Pentateuch from the Documentary Hypothesis viewpoint, one can accept the fact that Moses was not the sole author of this entire section of the Bible, as previously believed.  It also gives us deeper insight in the meaning behind various verses if one can understand the background and viewpoint of that particular "author" or school of authorship.  It must be noted that more conservative scholars vehemently reject this theory, saying that it shows a complete misunderstanding of how early writings were passed on, and "rewrites the Bible" by forcing a 19th century and later editing process on material that is ancient.