The Old Testament and the New Testament of The Bible are related.
The Old Testament Hebrew Scriptures are the collection of books that forms the first of the two-part Christian Biblical canon. The contents of the Old Testament canon vary from church to church, with the Orthodox communion having 51 books: the shared books are those of the shortest canon, that of the major Protestant communions, with 39 books.
The Old Testament of The Bible, is a history and accounting of the creation of the world and the history of the Jews, God's chosen people.
Although it is not a history book in the modern sense, the Old Testament is the primary source for the History of ancient Israel and Judah.
The tone is often dark in the sense that man sinned, and sacrifices of animals were the only thing to atone for their wrong doings. There is also a sense of revenge: an eye for an eye, and a tooth for a tooth—paying back one's enemies.
The Old Testament provides genealogies, and historical events, including wars, and the nomadic existence of the Jewish people. There are many stories about men (and women) of God within the Jewish culture who were often great leaders and/or faithful servants. There are prophecies, as well, with regard to the coming of the Jew's Messiah, foretold hundreds and hundreds of years before the birth of Christ.
The New Testament has a different tone. Based upon the coming of Christ, their is a new sense of hope, as well as a new ideology: turn the other cheek, more than once if necessary.
The New Testament...is the name given to the second major division of the Christian Bible, the first such division being the much longer Old Testament.
Unlike the Old Testament, the contents of the New Testament deal explicitly with Christianity, although both the Old and New Testament are regarded, together, as Sacred Scripture. The New Testament has therefore (in whole or in part) frequently accompanied the spread of Christianity around the world, and both reflects and serves as a source for Christian theology.
The New Testament begins with the birth of Jesus, to Mary and Joseph, and continues through to his teachings, and subsequent death at approximately the age of 33, crucified by the Romans, at the request of a zealous group of Jews who asked for the release of a prisoner, that Jesus would take his place. All of this was foretold in the Old Testament, but ironically, the Jews did not accept Jesus as their Messiah.
On the other hand, the New Testament does not only tell of the birth, life and death of Christ, but is also offers up a new church, founded on the teachings of Jesus, and made up of ardent followers from all nations and cultures. The books within the New Testament record events of the new church and offer advice on how to live Godly lives. There are prophecies, also, in the New Testament about the "end of days."