The Bible is a collection of writings by lots of different people written over maybe a thousand years, from a number of centuries before Jesus to a century after Jesus. I often like to refer to it as "the Scriptures" to make that point about it being lots of writings that were originally separate. What these writings have in common is that "the Old Testament" is writings that grabbed the Jewish people; writings that convinced them that they were God's word to them. And "the New Testament" is writings that grabbed people who believed in Jesus in the same way.