The Protestant Reformation started in the sixteenth century. A group of people began to protest the corruption in the Catholic church. Let me give you a sketch of some of the milestones of the Reformation.
First, Martin Luther was a pious monk that began to have a crisis of conscience. When this took place, he began to wrestle with his personal faith and he concluded that a man was justified by faith alone in Christ apart from the sacramental theology of the church. This led him to post ninety-five theses, which challenged Catholic theology.
Second, John Calvin in Geneva also began to question the Catholic church. However, he was much more systematic in his thinking. He created a robust Protestantism, which would last until today. His Institutes of Religionare still widely read.
Finally, in England Henry VIII broke off the church due to political reason, as he wanted a divorce, which the Catholic church would not grant. When this happened he broke off. In time there would be many great Protestant leaders, like Thomas Crammer.
In a word, the Protestant Reformation was a religious movement that challenged the Catholic church and caused a split within the church. I will add a link to a great book on this topic.