Here is an old archived post of mine; you might find it helpful.
The Jewish Bible contains the same 66 books as the Protestant Old Testament, for reasons I'll address in a minute. The Catholic Old Testament, on the other hand, contains 73 (with some additional chapters to both Daniel and Esther). The Orthodox Old Testament has the same 73 books as the Catholic version, and some Orthodox bodies have 75.
The reason for these discrepancies has to do with linguistic, theological, political, and historical concerns. (As LilyLamb can tell you from my posts on other boards, I tend to be long winded, and people's eyes glaze over reading my babblings, so I'll try to be brief. )
In the 2nd century B.C., Ptolemy, the king of Egypt, decided he wanted to build the greatest library in the world, which would contain a copy of every book ever written, all of them translated into Greek, which was the dominant language in that part of the world at the time. This would include, of course, the Jewish Scriptures. In Alexandria, there was a huge diasporic Jewish community, and seventy Jewish scholars were hired from that community to locate, gather, and translate every last book of Jewish Scripture that could be found. This was accomplished, and the name of this Greek translation of the Jewish Scriptures was the Septuagint, after the Latin word septus, meaning seventy---since seventy scholars worked on it. This Greek translation carried the 73 books currently found in Catholic and Orthodox Bibles.
By the time of Christ, the Septuagint had become accepted by many diasporic Jews throughout the ancient world; it was, however, not held in favor with the "legalistic" factions in Jerusalem, which would include the Pharisees and the Sadducees. To these folks, Hebrew was a sacred language, and in their way of thinking, if God wanted us to know something, then He'd see to it that it remained in the sacred language of Hebrew; and if God didn't care enough about a book remaining in Hebrew, then apparently it wasn't terribly important to begin with. Accordingly, they put together their own collection of the Jewish Scriptures, made from only the books still extant in the original Hebrew. (Another reason for rejecting the Septuagint books had to do with the fact that the Books of Maccabees contained evidence of friendship treaties between the Jews and the Romans---and by this time, the Jews had come to hate the Romans so much that they didn't want anything to do with them.) This Hebrew collection was called the Masoretic text, or sometimes the Masora. It contained the 66 books now found in all Jewish Bibles, since it rejected the Greek books found in the Septuagint.
It should be mentioned at this point that many copies of the Hebrew Scriptures had become lost or destroyed in the years between 300 B.C. and 200 A.D.; and Hebrew originals for some of these rejected books were impossible to find. Since the late 19th century, however, archaeological finds have uncovered copies of nearly all these books, or parts of them, in Hebrew, which were hidden by various people to protect them from being destroyed. So to use the reasoning of the Masoretic faction, God did preserve these books in Hebrew---it's just that they couldn't find them at the time.
By this time, the Christians had come along, and they tended to use the Septuagint, rather than the Masora. As time went on, they added their own writings (Gospels, epistles) to the corpus, and by the time of Pope Damasus, the whole works was translated into Latin by Jerome. Since Jerome was usuing regular street Latin instead of high-falutin' classical Latin, the Latin version was called the "Vulgate", after "vulgar" Latin. This became "the" Bible for Christians right up into the 16th century.
It must be borne in mind that during the first 400 years of Christian history, there was no clear-cut "canon" for Biblical books; there were many, many books produced during this period, some of them heretical (the Gnostic Gospel of Thomas, for example), some of them unorthodox (like the "infancy narratives" such as the Gospel of Psuedo-Matthew and the Proto-Gospel of James), and some of them orthodox but incomplete (such as the Didache). Some of these books were held as divinely inspired Scripture by some Christian communities, while some of the books in our present canon were rejected. The list which we now have (for the New Testament) was finally settled at the Council of Hippo in 393 A.D.; the same list was reconfirmed at the Councils of Carthage (397 and 418 A.D.), Florence (1441 A.D.), and Trent (1546 A.D.)
For a list of some of these extra-biblical books (both Jewish and Christian), go to
www.bible2000.org/forgottenindex.htm or wesley.nnu.edu/noncanon/. You will be astounded at the number of ancient writings out there which never made it into the Bible (and usually for good reason). Some of the New Testament writers were familiar with these books, and even quoted them in our Bible; for example, Matthew 7:13 is an echo of the Epistle of Barnabas 18-20; Matthew 14:13-21 is an echo of 2nd Baruch 29:8. Jude loved the Jewish apocrypha---verse 6 can be found in the Book of Enoch, verse 7 in the Testament of the 12 Patriarchs, and verse 9 can be found in the Assumption of Moses. (Of course, on the other hand, Paul himself also quoted from the pagan Greek poets Epimenides, Aratus, Menander, and Cleanthes.)
Anyway, the Greek Septuagint/Latin Vulgate was used as the standard Christian Bible right up to the time of the Reformation. When Luther came along, as we all know, he had some serious problems with certain Catholic doctrines such as purgatory. He rejected this doctrine, and in order to reinforce that rejection, he also rejected the seven Old Testament books from the Septuagint, since one of them (2nd Maccabees) contained a passage which corroborated the concept of purgatory. (Luther also had a problem with the concept of "works", and wanted to throw out the Epistle of James as well ["faith without works is dead"], but his friend Philip Melanchthon convinced him that if he kept on tossing books at the rate he was going, he was going to end up with a pretty thin Bible.) The other Reformers picked up on Luther's German translation with its omissions and additions (again, to reinforce his idea of sola fide, Luther added the word "alone" to Romans 5:1, changing it from "justified by faith" to "justified by faith alone"), and thus, all Protestant Bibles to the present day have 66 books, with the omission of the seven Old Testament books from the Greek Septuagint.
Catholic Bibles still contain those seven books, which consist of Judith, Tobit, Wisdom, Sirach, Baruch, and 1st and 2nd Maccabees; Orthodox Bibles contain them as well, and some Orthodox churches also accept 3rd and 4th Maccabees, for a total of 75 books.