There are about 1.6 billion Catholics on the planet, making it the biggest religion by some hundreds of millions. The Catholic Church has an almost 2000-year history, dating back to the first pope, Saint Peter himself (he's the bouncer you'll meet at the pearly gates and must get past in order to get in). The history of the church has been scarred by schisms, including the Reformation begun by monk Martin Luther, when splinter groups broke off and then grew into rival Christian denominations such as Anglicans, Presbyterians, Protestants, Puritans and today's Evangelicals. However, the Catholic Church was undeterred, expanding into new territories in advance - or sometimes in the rearguard - of the new age of Empires. As Spain and France in particular extended their reach, so did the church, converting much of Africa and almost all of South America. Meanwhile Catholics in the USA began as a smaller group, though now they are 70 million strong - that's nearly a third of the population.
Despite the profoundly hierarchical nature of the church, being Catholic means different things to different people: the vast majority of Catholics in developed countries use contraception, for example. However, whether they go to every service or not, keep all the rules or not, or accept all the elements of the theology or not, they are all part of the Church - more or less! Let's see how you fit in.