The Old Testament is mainly a collection of the faith stories of the Israelite people of ancient times, starting with their origins as a people (and the origins of the world), chronicling their escape from slavery in Egypt (amidst all those nasty plagues), all the way through the time of their kings and prophets to the exile in Babylon and then on to the revolt of the Maccabees brothers against the Greek authorities. The Old Testament (known in Jewish circles as “the Bible” or Tanak) is considered sacred to the Jewish people who have their own traditions about interpreting it. Christians interpret on its own terms but also in the light of the New Testament.
There is some disagreement about what belongs in the Old Testament. For Catholics there are 46 books; Protestants and Jews generally agree on 39; and for Orthodox Christians, there are 49-50 books (and an appendix). The controversial books are called “deutero-canonical” by the Catholics and Orthodox and “apocryphal” by the Protestants, who consider them important though not necessarily inspired. The books Catholics and Orthodox include were rejected by Protestants and Jews because they were not originally written in Hebrew, the traditional language of the ancient Israelites.