A Saint is someone who has been declared by the Christian Church to be an example of holiness and virtue that believers can follow. Any believer can pray for any saint's guidance and inspiration.
A Patron Saint is a saint chosen as a special guardian and protector by a person, institution or group.
Sometimes this is done through historical association with something in the saint's life or legend. Saints Cosmas and Damian, who were twins and healers, are patron saints of doctors and surgeons (though not of twins!) The angel Gabriel, God's messenger, is the patron saint of messengers, including broadcasters. The reason can be grisly - St. Lawrence, who was roasted alive on a gridiron, is the patron saint of cooks.
A saint can also become a patron saint by personal choice. A church makes St. John the Evangelist their patron saint when they dedicate the church to him. Parents make St. Francis Xavier their son's patron saint when they name the child after him.
Sometimes there's no explanation for a choice of patron. Brussels chose Saint Michael the Archangel as their patron and has him defeating the devil on the city coat of arms. That makes sense - he's a fine, strong protector for a city, and you can understand why he's also patron saint of swordsmen and soldiers - but why is he the patron saint of bakers and greengrocers?
The legends and lives of the saints make fascinating reading and often tell us more about the time in which they were written and the aspirations of the writers than about the saint him or herself. Saints have also inspired much great art and music.