The English Reformation started in the reign of Henry VIII.
Henry wanted to get rid of his first wife Catherine of Aragon because she had failed to produce a male heir to the throne.
However, getting divorced was not so simple as it is today. Henry was a Roman Catholic, and the head of this church was the Pope. According to the Roman Catholic belief, marriage was for life, divorce was forbidden.
Henry found himself in a complicated position. He made a special petition to the pope so that he might get a "Papal Dispensation", this meant that the pope would agree to grant the King of England the divorce, but not to others. Anyway, the Pope refused this petition. The King got so angry that he ordered the Archbishop of Canterbury to grant him the divorce so he could marry Ann Boleyn.
The Archbishop (in order to remain in good terms with Henry) granted him the divorce against the opinion of the Pope.
This event lead to England breaking away from the Roman Catholic Church based in Rome. Henry created his own church, placing himself as the head of that church, and then, his divorce was absolutely legal.
However, the English people did not react so well to this. In fact, they were very angry at the way the Roman Catholic Church used to spent their money. Common people had to paid for everything, if they wanted to get married, they had to pay. If they wanted to baptise their children, they had to pay. If they wanted to bury someone, they had to pay. So the church was utterly rich. With the reformation, all this injustices were exposed and the Catholic Church lost his popularity.