The head of the Roman Catholic church, elected by cardinals of that church. Normally celibate, although some notable exceptions occur to this during the medieval period.
During the medieval period, the Pope could have enormous power (see crusade). Theoretically, there is only one Pope. However, several times during the middle medieval ages there were multiple Popes, each of which claimed they were the legitimate and rightful Pope. The ones who eventually lost are known as Antipopes.
The modern Pope lives in Vatican, a religious city state in Italy, but originally the Pope lived in Rome (I believe), although a few Popes and Antipopes set up their capital elsewhere. At some stage (probably during the medieval period) the Pope settled in the Vatican for good.