The first five books of the Bible, Genesis to Deuteronomy, go by several names. One of them is 'The Law', a shorthand for 'The Law of Moses'. These five books are traditionally associated with Moses, and whether or not they were written entirely by him (it's unlikely, for instance, that Moses wrote the account of his own death in Deuteronomy), he is certainly the predominant human character in four of the five books. The Lord Jesus Christ and the New Testament writers refer to these books as 'The Law', and also associate them with Moses. Some take these references as indications that Moses was the predominant writer, while others feel that this is more of a convenient shorthand for referring to these books because Moses was so involved in the composition of so much of them.
The Hebrew equivalent of the word 'law' is the word torah, and in the Hebrew Bible this is the name given to these five books. The Hebrew Bible (Old Testament) as a whole is known as the Tanakh, and that term is an acronym for the three sections that comprise it: Torah (Law), Neviim (Prophets), and Ketuvim (Writings) - T-N-K or Tanakh.
Another name commonly given to these books is the term 'Pentateuch'. This comes from the idea of five (penta = five) volumes or books. Tthere are five books Genesis through Deuteronomy.
Not just law
There is a lot of legal material in these books precisely because they include the instructions God gave to Moses on Mount Sinai. These instructions formed the basis of the constitution of the nation of Israel once they left the land of Egypt and became God's people. Some of the laws are presented in a 'thou shalt not...' form, while others are set in a more circumstantial style: 'if such and such happens, this is what you should do...'. Still other parts of the Law are cast in the form of more general ethical or moral teaching.
There is also a lot of ritual material. The Law of Moses involved the creation of the tabernacle, a sort of tent or mobile-home in which the presence of God was enshrined as He journeyed with His people towards their land. He was in their midst, as it were - in the very centre of their camp. But how can a God of such holiness and purity be present among a sinful people? The answer is through a serious of ritual and sacrifices which were also prescribed. These, too, are detailed in the Law of Moses.
But there is a lot of other material in the Law as well which is not 'law' or ritual at all. The book of Genesis starts at creation and continues to the life of Joiseph and the the Israelites in the land of Egypt. The first half of Exodus is likewise Biblical history of how the Israelites came out of Egypt. A decent chunk of Numbers and Deuteronomy is also history. So the term 'Law' needs to be understood in quite a wide sense!
Finally, a few words perhaps ought to be said about so-called 'Pentateuchal criticism'. The term refers to an approach to the Bible which became popular in the 19th Century and beyond and which claimed to identify several different sources allegedly dating from quite different periods which were woven together to form our current Pentateuch. These alleged sources were given names like the Yahwist, the Elohist, the Priestly writer, and so on.
While this approach once seemed the chief modus operandi of most Biblical scholars, this is no longer the case. While it could be true that several different sources could have been used and edited to form the Law in its current form, it's interesting that scholars could never agree on exactly where the supposed divisions fell in many cases (and there was scant evidence for the various dating proposals). This ended up casting significant doubt on the value of the whole approach. Further, it has been increasingly realised that the Law in its current form has been put together in an extremely artful and skilled way, and that there is probably more value in studying it in its final form (even in the academy) than in attempting to break it down into little pieces which could never be agreed upon.
The fact is that if one believes in the divine inspiration of the Scriptures, then it is the final form which, under inspiration, God has chosen to preserve for all time. Whether or not this or that source ever existed, it certainly does not exist any more - whereas the Bible in its current form, in all its beauty and wonder, certainly does. The connections, integration, and flow of themes, history, and ideas in the Law of Moses is one of its chief wonders.
Q: Questions coming soon!
A: And answers too...!