Back in the day (the fifteenth century), the Church was a pretty big part of everyday life. Back then, there were no denominations of Christianty: if you were a Christian, you were a Catholic. Most, if not all, countries incorportated religion into their laws. This often meant paying a sum of your income to the Church every year (called tithes). The Church also did some other questionable things, such as selling indulgences to people, which basically said that if you paid a certain amount of money, you would get time off your sentence in purgatory.
The other countries of Europe (aka not Rome) started to become a little uneasy with the tithes. They noticed that everything, not just the churches, in Rome was getting nicer. They soon put two and two together and realized that all of this money that was going to Rome was from their citizens. They could be benefitting from the profit. They weren't very happy. But, because they didn't want to anger God, they couldn't tell the pope he was wrong due to ex cathedra (literally speaking from the seat of Peter {in the Bible, Jesus promised Peter that it was impossible for the Pope to say something false or incorrect while seated on his papal throne]).
How did the other countries combat this theft? By nominating their own Pope, of course! There was still a Pope in the Church, but other countries elected a Pope from the French city of Avignon. Some of the countries were obviously not too thrilled about this, but enough countries backed him up that the disagreeing countries couldn't really do anything about it. So, for a number of years, there were two Popes.
Eventually, the Romans decided that enough was enough and elected another person to take the place of both Popes. However, when he got to the Vatican, the other two popes refused to acknowledge him as Pope, everyone claimed that they were the rightful Pope, and basically excommunicated each other. So now there were three Popes and no solution in sight.
Eventually, the Council of Constance was called by one of the Popes and they got two of the Popes to agree to resign but sadly had to excommunicate the third. They then elected a new pope, the Orthodox Church decided to split from the Catholic Church (because it was an independent Church who don't need no Pope), and, as they say, the rest is history.
This event definitely shows that human beings are and have been stubborn, a prime example being the Pope that had to be excommunicated. If people really want something, they'll stick with it, no matter what it takes.