On the one hand, the Catholic canon (list of sacred books) for the Old Testament follows the canon as practiced within the early Christian church. On the other hand, the Protestant Bible follows the canon developed by the Jewish community in ancient times.
The early Christian church primarily used the Septuagint of the Hebrew Bible, a Greek translation of the Hebrew Bible, and some additional writings that were primarily written within the Greek-speaking Jewish community of the diaspora (Jews residing outside of the Holy Land).
The Christian canon was developed and ratified throughout the centuries by several church councils, but sixteenth century Protestant reformers chose to include in their Bibles only the books of the Hebrew Bible (TaNaKh).
The additional books included in the Catholic Bible are called the deuterocanonical (secondary canon) books by Catholics. They are called Apocrypha (hidden) books by most Protestants, and they include: Wisdom of Ben Sira, Wisdom of Solomon, 1 Maccabees, 2 Maccabees, Baruch, Tobit, Judith, and some parts that were added in the Greek translations of the canonical books of Daniel and Esther.
The difference in the number or books in the Bible is only found in the Old Testament. All Christians profess the same 27 books of the New Testament. Can you name them? Have fun trying!