When the early Christians started organizing and had counsils, such as the Council of Nicea and Synod of Hippo. they accepted certain books and eliminated others. There were Gospels that were kept out that they felt didn't have enough evidence to support them, or weren't influenced by God. Likewise, the New Testament is made up of letters written mostly by St. Paul as well as the Catholic letters such as Saints Peter and James that were accepted. Most of that was based on if they felt they were actually written by those authors or others.
The Catholic Bible has 73 books, while the non Catholic has 66. All of the New Testament is the same, the difference is some books of the Old Testament.