@Jeremiah,
At the time the King James was made, several books were dropped as they tended to promote the Catholic view of things. These are usually referred to as the apocryphal books.
The main differences in Catholicism and protestantism are theological. Catholics believe the miracle of the transubstantiation, that during mass the bread and wine literally become the body and blood of Jesus in all important aspects. Protestants tend to believe this is symbolic, and that the bread is still bread, the wine, still wine.
Many protestants go with just the Bible as their source of faith. "If it isn't in the bible, then it's not divine word." Catholics also respect holy tradition as a manifestation of God's Will. In ways, the Catholic Version is more American, as we don't just go by the book in our legal system, but also allow for precedent and common practices which may not be recorded.
Hope this helps!