Just so you know.... this thread is going to have a huge potential to get nasty. This is another point of contention between Catholics and Protestants/other Christians.
So... Here's what happened, to my knowledge (I'm not Catholic by the way)
The extra books were written AFTER the books considered as the Jewish Bible but BEFORE the New Testament. The Jewish Bible is called the Tanakh. It contains the 21 books of the Old Testament that are also found in the King James version.
The Tanakh was compiled in 450 BC. Now in the third century BC, the Hebrew Bible was translated into Greek. This is known as the Septuagint. The Septuagint contains the extra books:
Tobit, Judith, Wisdom of Solomon, Wisdom of Jesus Sirach, Baruch, Epistle of Jeremy (which later became chapter 6 of Baruch in the Vulgate), additions to Daniel (The Prayer of Azarias, the Song of the Three Children, Sosanna and Bel and the Dragon), additions to Esther, 1 Maccabees, 2 Maccabees, 3 Maccabees, 4 Maccabees, 1 Esdras, Odes, including the Prayer of Manasses, and Psalm 151
Many of those books were never written in Hebrew and were written in Greek at the time of compilation of the Septuagint. The books are still not recognized as part of the Jewish Bible (The Tanakh). So, Protestant Churches have taken the conservative route and only accepted what Judaism has traditionally accepted. Catholics and Orthodox Churches believe that the NT makes reference to some of those extra books so they include them.
Now, I believe the Catholics use some of the extra books in their Bible, and the Eastern Orthodox use all of them.
Personally, I am conservative here, and I look at them as not inspired, but certainly good commentary.
On par with say, what C.S. Lewis has written.