A saint is:
1. a Christian who has lived a holy life in God's service which should serve as an example to other Christians
2. one who is declared by the Catholic church, through the process of canonization, to be in heaven
Because saints are in heaven and in good grace with God, Christians, especially Catholics, ask these saints to intercede for them in prayer to God. (This is not worship of the saint!) Due to the nature of each saint's life on earth, he or she is likely to take up certain causes, which are said to be that saint's patronage.
In short, a patron saint is a holy person in heaven who prays for those in certain situations on earth. Which situations are assigned to each saint is decided by the aspects of the saints' earthly lives.
1. St. Joseph is the patron saint of families because he was the patriarch of the Holy Family.
2. St. Mary is the patron saint of the Americas because of the apparition of Our Lady at Guadeloupe.