A saint is a person who has been recognized the Catholic Church or the Orthodox Church as doing great works or making a great personal sacrifice for the Christian church. In reality anyone who has made it to heaven is a saint. Persons recognized as saints must first be beatified (After which the deceased person is given the title 'Blessed') and then canonized by the Pope. Usually a person is canonized if at least three miracles have been performed by them through lay people asking the deceased to intercede with God on their behalf. In Catholic and Orthodox doctrine, saints are believed to be able to intercede on behalf of mankind before God. The Queen of Saints is Mary, the Mother of God or the Blessed Virgin.
Beatification and Canonization, http://www.newadvent.org/cathen/02364b.htm