The word TESTAMENT means PROMISE. And Jesus is the fulfillment of the Promise.
The OLD TESTAMNET tells us how God spoke to His people before Jesus came upon earth. There are four major divisions in the Old Testament according to the styles of writing and span over 900 years. The first five books are called the Pentateuch: Genesis, Exodus, Leviticus, Numbers, and Deuteronomy. The next division is comprised of the history books telling the history of the Israelites, from their conquest of Canaan to their defeat and exile in Babylon. Then there are the poetic and "Wisdom books" dealing, in various forms, with questions of good and evil in the world. And finally, the books of the biblical prophets, warning of the consequences of turning away from God.
THE NEW TESTAMENT tells us about the life, death, and resurrection and ascension of Jesus, and about the coming of the Holy Spirit. The first four books of the New Testament are called Gospels. The word "Gospel" means GOOD NEWS. Saints Matthew, Mark, Luke and John each wrote a Gospel.
When spreading the Good News, the Apostles did not remain in one place forever; therefore, as they moved from place to place, they wrote letters to the new Christians among whom they had worked just to remind them how they should follow Jesus by the kind of life they should now live. These letters are called Epistles. There are 21 in all. The New Testament also includes the Acts of the Apostles and the Book of Revelation. (Summarized from Book Four, God With Us Catechetical Series)