Some of God's truths can be known by human reason alone (such as the existence of God). But many truths of the Catholic faith are known only because God revealed them (CCC 50). God's revelation to mankind was given "through the prophets" (Heb. 1:1), and in the New Covenant, through Jesus and the Apostles (CCC 75). Revelation, or the "Word of God" (1 Thess. 2:13) is contained in Sacred Scripture (The Bible) and Sacred Tradition (CCC 82).
The Bible is a collection of 73 separate books (CCC 120), which are inspired (2 Tim. 3:16) and inerrant (Num. 23:19). These books were written under the inspiration of the Holy Spirit (CCC 688), so that the Biblical writers wrote exactly what God wanted them to write–nothing more, nothing less. This does not mean God dictated the exact words He wanted written, but rather, the writers used their own words, style, and writing skill (CCC 106). Therefore, the primary author of Scripture is God (CCC 105), though He used human agents to write it. Because God is the author of Scripture, the Bible is inerrant, meaning it contains no errors, and everything contained therein is true (CCC 107). This does not mean everything in Scripture is literal truth–Biblical accounts are sometimes parables and allegories.
Sacred Tradition is the teachings of Jesus and the Apostles (or the Word of God) passed down through a succession of bishops (CCC 81) by means of oral tradition, and as recorded in creeds, authoritative writings of Popes and bishops, and especially through the writings of early Christian writers and theologians commonly referred to as the "Fathers of the Church" (CCC 8). Whereas Sacred Scripture is the written Word of God, Sacred Tradition is the unwritten Word of God. Both, however, are equal in authority in the Church (CCC 80).
Although the fullness of truth contained in both Scripture and Tradition, some truths are contained only implicitly rather than explicitly. Public revelation ended with the death of the last apostle (John in c. 100 A.D.), but the "faith once for all delivered to the saints" (Jude 3) has gone through what has been called "doctrinal development". Doctrinal development refers to the process by which a revealed truth or doctrine, passed down from generation to generation, becomes understood more clearly and deeply–through prayer, study, and the light of the Holy Spirit–in the fullness of time (CCC 94). As a result, in latter eras of the Church, we see fuller explanations of doctrines and devotional practices that were unknown in earlier Christian eras. One must be careful to use the word "development" rather than the word "change". Doctrine does not, and in fact cannot, change, although inferences and implications are drawn from what is known, while obscurities are cleared up. Nor is it possible for a belief to be "invented" or "added". Everything the Church now teaches was contained in public revelation, although some were as "seedlings" that had not yet sprouted–they were present, although implicitly. Occasionally, new insights draw out what was implicitly believed, and are explicitly stated, as the Holy Spirit leads the Church into "the fullness of truth" (John 16:13). This is most clearly seen in the Church's Christological [concerning Christ] developments. Arius, in the year 318, denied that Jesus was God, and a Council was held (The Council of Nicaea) in order to define the doctrine regarding Christ's divinity in such detail as had not yet been explicity stated, because they had to think about Christ and his divine nature in a way that Christians never before had. A century or two later, Nestorians claimed Jesus was two persons, the Monophysites claimed Jesus had only a human nature, and the Monothelites claimed Jesus had only a divine will, not a human will. These heresies arose because the Church had not pondered these issues in any detail. But when these heresies challenged the Church, Councils were convened and the doctrines concerning Christ were laid out explicitly, in great detail. Even though these beliefs were always at least implicitly held, they were not spelled out as clearly as they were when these heresies were dealt with.
Because "In them [the epistles of Paul] there are some things hard to understand that the ignorant and unstable distort to their own destruction, just as they do the other scriptures" (2 Pet. 3:16), an infallible interpreter (of Scripture and Tradition) is needed, and that interpreter is the Magisterium (teaching office of the Church) (CCC 85). Scripture must be interpreted in the light of Tradition, and vice versa (CCC 113). The Magisterium is specifically enlightened by the Holy Spirit to do just this. Scripture, Tradition, and the Magisterium is like a tripod–each leg must be in place or the whole thing collapses.
Besides public revelation, certain people in the history of the Church have received private revelations, called so because they are usually given to a single individual. These revelations usually come from apparitions, visions, and locutions. However, since these revelations are not part of the deposit of faith, and are not incorporated into the official teachings of the Church, which was "once for all delivered to the saints" (Jude 3), none of these revelations are binding upon Catholics, even though the Church declares some of these revelations to be worthy of belief. Christians are free to believe these revelations (or free to reject them), unless they contradict the official teaching of the Church, because the latter we know is true, while the former we know is false if it disagrees with the teachings of the Church (CCC 67).