As a Catholic...
All Catholics are Christians, not all Christians are Catholic.
Of the estimated 2 billion Christians in the world. 1.2 billion or roughly 2/3rds are Catholic.
Catholicism was the original Christian faith and was the only, until around the 1500s when some Chrsitians, first led by Martin Luther, broke away from it and started up their own faith.
Today, there are over 30,000 different denominations of Christianity, yet still only 1 Catholic church.
The term was first applied in the 1st century, by a Bishop named Ignatius, who was taught by Peter, the apostle. In a letter to the Christians in Rome and Antioch, he urged them to follow "this Katholikos Church" -- Greek for Catholic Church. He referred to it by name as if it already existed even in that time.
The Catholic church was the first to make the Trinity a central doctrine of the Christian faith and defend it against Gnostic and Aryian heresies.
The Catholic church decided what would go into the Christian bible and canonized it, preserved it from errors until the 15th century when the Printing Press allowed for more accurate translations.
It was not until the 16th century that non-Catholic Christians who broke away changed the bible by removing 7 books from the Old Testament. It is this 16th century modified version that most non-Catholics use to this day.
Read the Nicene Creed developed in 325AD to defend against Aryian heresies which tried to claim that Jesus was not God
Basic Doctrines:
God exists in a Trinitarian form - 3 beings in 1 God
Jesus is God and came down to the Earth for the forgiveness of sins.
Individual Salvation depends on a person's "faith" in Jesus Christ - and by "faith" the Catholic church means following his teachings which involves an act of will and action.
Salvation as a whole means the process of avoiding sin and temptation, as well as ending one's life in friendship with God and going to heaven for an eternity. Salvation therefore has 2 benefits, a primary - eternal life, and a secondary - avoiding the many pitfalls of life by not sinning as much.
Salvation is by Grace. Grace, by definition, is an unmerited gift. All gifts must be accepted, they can not be forced. The way God wants us to accept his grace is by living a life of Love, Faith, and Good Works. -- All 3 being related to each other, and enhancing one another.
Faith leads to Love, Love demands expression (good deeds) Good works without love are done for the wrong reason. What we do, influences what we feel and think. What we feel and think influences what we do. That is why all 3, not just Faith, are necessary for salvation. They are the proper response that God desires from us to accept his grace.
The Bible is the word of God. However the bible is not the end of the story, but the beginning. It ends abruptly with the Apostles.. yet Christianity was just beginning. We should not consider the bible the ONLY source of information and authority regarding Christ.
Jesus instituted authority in his Apostles. Not a mere book. The bible is important, and none of our teachings should violate what it says, but it is not the sole source and end-all be-all regarding Christianity.
Christ instituted the Sacraments to help us and guide us. They are:
Baptism
Confession
Communion
Confirmation
Marriage
Holy Orders
Last Rites
Since the Sacraments were instituted by Christ, as evidenced by the bible, we can not change them or ignore them. They are essential and important.
We can ask anyone for help and to intercede on our behalf. This includes those in heaven, for they are not dead or cut-off from Christians, since both are linked by their faith in Jesus. If anything, those in heaven are more alive, since they are with God. The way one asks someone in heaven to intercede for them is through prayer.
Prayer is not worship, it is communication. We communicate most often with God, when we pray, but prayer itself is not worship and that is why we can use prayer to ask those in heaven to intercede on our behalf.
There is a difference between religious art and idols. One can use religious art to remind them of how we should act, or to help us think about God. Visual reminders are a good thing, they are not idols unless we actually worship them. Worship is not by accident, it is a definite act of will, something we deliberately do.