Who was William Penn?

William Penn was the founder of Pennsylvania. Most importantly, he was a convert to Quakerism, at that time a fast-growing and persecuted Christian denomination in the British Isles founded by George Fox. Charles II owed Penn's late father a good deal of money, so he gave Penn's surviving son a large tract of land in North America to settle what he owed.
Penn immediately founded a colony there. This helped alleviate two problems for Charles II: not only did the cash-strapped king get out from under debts, he created a safety valve that helped him get rid of Quakers, who were a nuisance to him. Charles found their refusal to pay tithes (taxes) to the Church of England, of which he was head, their refusal to swear oaths, and their refusal to accept hierarchy, believing they could communicate directly with God without the need of his priests, a threat to his reign. After years of persecution only seemed to swell their numbers, it was a boon, from Charles point of view, to ship many more of them across an ocean. (Some Quakers had already settled in New Jersey and Maryland.)
As with the Puritans, Penn hoped to establish a model colony based on primitive Biblical principles. Unlike the Puritans, however, Penn believed firmly in religious toleration, and even invited in non-Quaker sects that were persecuted in Europe, such as German and Swiss anabaptists, which was the beginning of the Pennsylvania Dutch (or Deutsch). Quakers were also pacifists, and Penn went out of his way to establish fair treaties with Native Americans and treat them decently. He decreed all trials be by jury, and, knowing what Quakers had suffered in British prisons, tried to make his colonial prisons humane. Some of Penn's doctrines, such as religious freedom, made their way in the founding documents of the United States.
Through Pennsylvania, Penn had a strong influence on the growth of America.