In this short but theologically heady article, a professor of the Old Testament at Midwestern Baptist Theological Seminary probes the history of the "Deuterocanonical" books in the Catholic Bible (Old Testament, to be more specific).
Let's break down that word before you click on the link to read his short history. "Deutero-" means second. "Canon" means an autoritative text. We Protestants call the extra books in the Catholic Bible the "Apocrypha." Historically, they are "Deuterocanon," not because those books are second in importance to the Catholic, but because they were received by the church after the original Hebrew books of the Old Testament were recognized as biblically canonical.
Whew!
With that out of the way, feel free to engage in a short, but rich, history of why Roman Catholics have these extra books. Access the article by clicking --> here <--