The Old Testament is a historical record of God's interaction with his specific chosen people.
However, almost everything that happens in the OT symbolizes and foreshadows (in God's way, because he has the power to symbolize events by other events) something that has happened in the time of Christ.
For example, the Israelites are enslaved in Egypt; God delivers them from Pharaoh miraculously by the plagues and the crossing of the Red Sea; they are led through the desert, fed on miraculous bread from heaven, yet complain, and as a result have to wander through the desert for forty years before being allowed to reach the Promised Land.
The Christian understanding of these events is, and this is not controversial among any denominations as far as I know: the slavery in Egypt represents every human's pre-baptismal slavery to sin and Satan; crossing the Red Sea represents Baptism; the miraculous bread from heaven, the manna, is Jesus Christ (who said explicitly, I am the bread of life); the complaints are the complaints and rebellions of Christians; wandering through the desert is the spiritual life; the Promised Land is Heaven.
All sorts of things in the Old Testament represent New Testament realities. The Ark of the Covenant, which in the OT a man was struck dead for touching, carried the tablets of the Ten Commandments (the word of God on stone), the rod of Aaron (a dead tree limb brought back to flower), and an urn of the manna from the desert. The Blessed Virgin Mary, who was never "touched" by a man, carried within her own body Jesus Christ, who is the Word of God made flesh (as opposed to written on stone), who died (like the tree limb) but was raised from the dead, who is himself the bread of life. So the Ark of the Covenant pretty obviously symbolizes the Blessed Virgin Mary.
A lot of the more violent parts of the OT make less sense if you don't understand that, although they are historical, their main value of edification comes from application to the spiritual life. Israel is God's chosen people in the OT: in the NT, God's chosen people is the Church that Jesus Christ established. So Israel represents the Church, and Israel's enemies are the enemies of the Church. For example, that famous Psalm where the Psalmist ends with something ridiculously violent, I think it goes like "Blessed is the man who takes your little ones and dashes them against the stones." Clearly that sounds a little too angry for a Christian to adopt, until you realize that the main application of it is spiritual; the "little ones" are the tiny beginnings of temptations to sin, and the stones represent the cornerstone of the Church, who is Jesus.
This kind of "typological" reading of the Old Testament is not something modern Christians have just made up: it's actually taught in the New Testament itself. Because we believe that both the Old and New Testaments have God for their primary author, we read it as a sort of a long story, written by God, and played out over thousands of years of real history. Obviously, the story has a really good twist with the Incarnation, death, and Resurrection of our Lord Jesus. That's kind of the central plot point of the entire story. So it only makes sense, for those of us who know how the story plays out, to go back to the first half of the story and read it with the big plot twist in mind, seeing where the author snuck in clever parallels and foreshadowings of what was going to happen right under the noses of those who came before us but failed to guess what was going to happen.