Before we can answer the question of what is a Patron Saint, we need to first deal with what is a saint.
The most basic definition of a saint is a being who is in heaven. This would include any angel and any human that died and whose soul has been allowed to enter into the heavenly kingdom. If the Catholic Church has "canonized" or "declared" a person to have lived (after an extensive, investigative process) an "exemplary and holy way" the person can be referred to as Saint (with an upper case "S").
A canonized Saint is sometimes "assigned" to be the Patron of a particular group of people (e.g., Saint Joseph of Cupertino - Patron of pilots) or against a particular illness (e.g., Saint Peregrine - Patron against cancer.)
Usually, the reason a Saint is listed for these particular subjects is related to something that happened during the Saint's life. For example, Saint Joseph of Cupertino was not an airplane pilot, but during periods of supernatural ecstasies, he would levitate or float. Saint Peregrine had cancer in his foot and was scheduled to have the foot amputated. The night before the surgery, he spent the night in prayer. The next morning, Peregrine's cancer was completely gone.
In future blogs, I will explore some of these Saints and the reasons why the Saints are patrons of particular subjects.