The Ten Commandments are a summary of God's Law. The Ten Commandments are more literally the Ten Words. Scripture is clear that the Words are ten in number but does not say how to divide them. Jews, Eastern Orthodox, Roman Catholic/Lutheran, and Protestants each have their own way of dividing the Ten Words. Here is a helpful table showing the differences. For reasons which may become more apparent when I explore the doctrine of Scripture, I will be following the Jewish method of division.
The Law has three uses. 1. The Law curbs sin in the civil realm. 2. The Law shows us our sin. 3. The law provides the Christian with a guide to regulate his life.
The first use of the law by itself doesn't save anyone from eternal damnation. The first use helps to maintain good order in society so that the Gospel can be preached freely. Also, the Law is still the Law. Disobedience against the law in the civil realm is disobedience against God. The Lamb paid for these civil sins because they are ultimately sins against God. The temporal punishments given out by earthly judges remind us that we will have to stand before the far more powerful Lamb who sits on the judgment throne.
The second use of the Law is the most important in Christian preaching--it drives us to the Lamb on the Altar. If God had not provided us with an explanation of His Law it would still be His Law, but we would be foolish enough to think that we are pretty good people. The Law shows us our need for the slain Lamb on the Altar. The second use of the Law shows us that we deserve God's temporal and eternal punishment.
The third use of the law is as a guide to Christian living and for doing good works to help our neighbor. We never attain perfection--we continually struggle against our sinful nature. We never outgrow our need for the second use of the law--we sin daily. But the blood of the Lamb works in us and we do grow ever so little in holiness. Through our growth we are driven back to the second use of the law as we become more and more aware of our own sin and more and more aware of our need for the Lamb on the Altar. Through the sacrifice of the Lamb on the Altar, in a relatively insignificant way we learn to sacrifice ourselves for our neighbor.