See this previous posting.
Why Do You Obey the Pope?
Catholics believe that Jesus commissioned St. Peter as the first leader of the Church.
Matthew's Gospel has the most direct biblical indication of the papacy: "And I tell you, you are Peter [meaning literally "Rock"], and on this rock I will build my church...I will give you the keys of the kingdom of heaven" (Mt 16:18-19). Based on this statement of Jesus himself, Peter is clearly portrayed in the New Testament as the leader of the disciples.
A pope can make infallible, binding pronouncements under certain conditions. Infallibility doesn't mean that absolutely everything a pope says is free from error. All Christians believe that God protected Holy Scriptures from error, by means of inspiration, even though sinful, fallible men wrote it.
We Catholics also believe that God the Holy Spirit protects His Church and its head from error (John 14:16), by means of infallibility, even though imperfect men are involved in it.
Other good resources dealing with this same question:
Downloadable mp3 entitled Apostolic Authority and the Pope