What do Catholics believe? How does it differ from Christianity? What books do they use besides the Bible? What do these books teach and where did they come from?

his could be a massive answer; entire books have been written on the distinctives of Catholic faith and doctrine. And Catholicism is considered a member of the Christian faith so, with that in mind, we will look at a few major differences between Catholic doctrine and Biblical Christianity:
(1) One of the main points that came out of the Reformation was the idea of sola scriptura—“Scripture alone.” In other words, all doctrines and teachings should be based strictly on what the Bible teaches. Beginning with the Council of Trent (mid-16th century) and affirmed by Vatican II (1963-65), the Church affirmed its right to interpret what the Bible says. The Catholic Church has developed over 150 volumes of “Sacred Traditions” (almost all coming after the first century) that it teaches are the equal of Biblical scripture because apostolic authority has been passed on to all succeeding bishops throughout the centuries—meaning that the Pope today speaks with the same authority as did Paul and the rest of the apostles in the first century. Catholic scholars teach that “the Church is always advancing towards . . . divine truth.” Therefore, Catholics teaching the Bible “plus” their traditions as valid church authority.
(2) Catholics believe that the Bible is a “church-based book” because the Church wrote or determined what books make up the New Testament. Biblical Christianity would say that the New Testament was “discovered” by means of the Holy Spirit, the Holy Spirit making clear what books were appropriate for inclusion in the New Testament. As Peter said so clearly in his second letter, Above all, you must understand that no prophecy of Scripture came about by the prophet’s own interpretation. For prophecy never had its origin in the will of man, but men spoke from God as they were carried along by the Holy Spirit—II Peter 1:20-21. That is one reason why Protestant Bibles do not include the books of Tobit, Judith, 1 and 2 Maccabees (historical books), Wisdom, Sirach (wisdom books), and Baruch (prophetic book), plus parts of Esther (10:4-16:24) and parts of Daniel (3:24-90 along with chapters 13-14). Also, these books were not considered part of the Jewish canon (books considered by the Jews to be divinely inspired) and therefore they were not considered part of the Protestant Bible.
(3) Biblical Christianity teaches that, as Paul wrote, For there is one God and one mediator between God and men, the man Christ Jesus, who gave himself as a ransom for all men—the testimony given in its proper time—I Timothy 2:5-6. Catholics teach the need for a priest to intercede on behalf of people. The Bible clearly teaches otherwise; as the writer of Hebrews said, Therefore, since we have a great high priest who has gone through the heavens, Jesus the Son of God, let us hold firmly to the faith we profess. For we do not have a high priest who is unable to sympathize with our weaknesses, but we have one who has been tempted in every way, just as we are—yet was without sin. Let us then approach the throne of grace with confidence, so that we may receive mercy and find grace to help us in our time of need—Hebrews 4:14-16.
(4) Catholics believe that, as much as Christ’s blood atones for our sins, it is not enough; we must also show works that continue the work that Christ’s death began. In fact, the Church’s own Council of Trent (1545-1563) maybe says it clearest, that anyone believing the sacraments of the Roman Catholic Church were not necessary for salvation and they could obtain justification through faith alone was anathema (cursed). This is what got Martin Luther excommunicated from the church. However, the scriptures clearly teach this. As Paul said so famously to the church at Ephesus, For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God—not by works, so that no one can boast—Paul, Ephesians 2:8-9. He repeated this idea in a pastoral letter to Titus, where he said, But when the kindness and love of God our Savior appeared, he saved us, not because of righteous things we have done, but because of his mercy. He saved us through the washing of rebirth and renewal by the Holy Spirit, whom he poured out on us generously through Jesus Christ our Savior, so that, having been justified by his grace, we might become heirs having the hope of eternal life—Paul, Titus 3:4-7. And as Paul declared so emphatically in his letter, ironically, to the church at Rome, Therefore, since we have been justified through faith, we have peace with God through our Lord Jesus Christ, through whom we have gained access by faith into this grace in which we now stand. And we rejoice in the hope of the glory of God—Romans 5:1-2.
(5) The Catholic Church teaches a belief in purgatory—not hell, but a place of final consequence of sin where payment for the sins of a person is completed and believers are then ready for heaven. They believe this because they feel it is impossible to pay for all of one’s sins during a lifetime, obviously making the point that payment for sin still lies with the believer. The Bible would say otherwise; as the author of Hebrews explains, For Christ did not enter a man-made sanctuary that was only a copy of the true one; he entered heaven itself, now to appear for us in God’s presence. Nor did he enter heaven to offer himself again and again, the way the high priest enters the Most Holy Place every year with blood that is not his own. Then Christ would have had to suffer many times since the creation of the world. But now he has appeared once for all at the end of the ages to do away with sin by the sacrifice of himself. Just as man is destined to die once, and after that to face judgment, so Christ was sacrificed once to take away the sins of many people; and he will appear a second time, not to bear sin, but to bring salvation to those who are waiting for him—Hebrews 9:24-28. And how much sin was covered by Christ’s blood? The apostle John tells us: But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus, his Son, purifies us from all sin—I John 1:7.
Ultimately, any time we add our efforts to Christ, we make Him smaller. He did ALL for us, for ALL time. ALL our sins are gone. ALL our sins are forgiven. There is nothing left for me to do. Everything that needed to be done for my salvation, Jesus did—wholly and completely. Again, as the author of Hebrews tells it, Now there have been many of those priests, since death prevented them from continuing in office; but because Jesus lives forever, he has a permanent priesthood. Therefore he is able to save completely those who come to God through him, because he always lives to intercede for them. Such a high priest truly meets our need—one who is holy, blameless, pure, set apart from sinners, exalted above the heavens. Unlike the other high priests, he does not need to offer sacrifices day after day, first for his own sins, and then for the sins of the people. He sacrificed for their sins once for all when he offered himself—Hebrews 7:23-27.