Catholicism is a denomination, hence a subset, of Christianity. All Catholics are Christians, but not all Christians are Catholics. A Christian refers to a follower of Jesus Christ who may be a Catholic, Protestant, Gnostic, Mormon, Evangelical, Anglican or Orthodox, or follower of another branch of the religion.
A Catholic refers to a Christian who follows the Catholic religion as transmitted through the succession of Popes of Rome and the Vatican Empire across history. The Pope is the leader of the Catholic church. The Catholic Church is the largest of the Christian Churches - about 60% of Christians are Catholic.
Catholicism is the original form of Christianity, all other christian religions broke off from the Catholic Church directly and by proxy in some cases, but historically, the origin of all the christian religions in the world can be traced back to Catholicism, the religion which Christ himself founded. the reason christian religions aside from Catholicism exist is due to the fact that in the early 1500s, Martin Luther, a Catholic Monk, decided that he would reject Church authority, and make himself his own authority, creating his own doctrines, and his own religion, and even altering the Bible to suit his purposes, th result of this was Lutheranism, and as a domino effect, it gave way to anglicanism in England, and other protestant religons broke off from them, and so forth, until we ended up with the splintered 30,000 christian religions in the world, that all use an incomplete bible that was authored by Martin Luther. and nobody can legitimatly say that Luther was atteempting to "reform" things, he said so himself, in his own writings, that he was doing this, only so he could make himself his own authority on all things
It's like saying what's the difference between a Golden retriever and a dog. Christianity refers to all religious denominations that believe in the trinity concept and Jesus as the savior. Catholicism is just one of those.
It's true as mentioned, Catholics are Christians. There are all kinds of differences between all denominations of Christianity. Just to mention one- Catholics place particular reverence to the Virgin Mary in their rituals compared to other denominations. I believe one thing all Christians firmly believe in is the "Gospel "which refers to Christ's virgin birth, his crucifixion , and his resurrection.
Simply put, true Christianity, which began outside the gate of the Garden of Eden, believes in the validity of the Bible considering it to be the Holy Word of God and patterning their lives after its teachings. The Roman Catholic Church teaches that the Pope is the final arbiter of truth and its members must pattern their lives after his teachings and interpretations of the Bible. It's man's wisdom vs God's wisdom