WHAT'S THE DIFFERENCE BETWEEN THE OLD AND NEW TESTAMENT (COVENANT)?
The Old and New testaments really are unified, meaning complementary, in a whole host of ways. Since the Old was first written it is the foundation for both. The Old established certain principles that would be seen in the New. For instance there were many prophecies spoken in the Old by the prophets that would be fulfilled in the New.
The Old established a complete history of God's people, a Nation. We see God's wrath against sin in the Old Testament, while the New shows the grace of God towards sinners. We see that the Old predicted a messiah (Jesus), see Isaiah chapter 53, and the New Testament describes and identifies Him in John 4:25-26. Furthermore, the Old described the established law and the New fulfills the law.
The Old dealt mainly with the Jews and the new dealings are for His Church, which includes everyone who accepts JESUS as their savior. God continues to bring revelation to the Church today as JESUS is the high priest, and the New clarifies to man the need for salvation. Both Testaments shows a righteousness God who condemns sin but desires to save sinners through atoning sacrifice, in both Testaments. This overview is short compared to what one could study, but should give the reader a basis for understanding.