Short answers, pulled from the story.
Cardinals are senior clergy who advise the pope and participate in the Church's central governance through papal consistories. Their most solemn duty is to elect a new pope in a conclave when the Holy See is vacant. During a vacant papacy, the day-to-day governance of the Holy See passes to the College of Cardinals.
Cardinals are chosen entirely by the pope, with no strict criteria and tradition as his only guide. Since 1917, a candidate must already be at least a priest; since 1962, those who are not already bishops must generally receive episcopal consecration. A small number receive a papal dispensation from the episcopal requirement.
Cardinals who have not yet turned 80 on the day of the vacancy are eligible to vote in a papal conclave. Pope Paul VI's 1975 document Romano Pontifici eligendo set the maximum number of electors at 120. The conclave following the death of Pope Francis in 2025 saw 133 cardinals participate, exceeding that ceiling.
The scarlet garments worn by Catholic cardinals symbolize a willingness to die for the faith. In choir dress, scarlet covers the cassock, mozzetta, and biretta, worn over a white rochet and a scarlet zucchetto. Pope Innocent IV granted cardinals the privilege of wearing the red hat in 1244.
Henry of Portugal is the only cardinal in history to have been crowned king, ascending to the Portuguese throne due to a succession crisis. John II Casimir Vasa resigned the cardinalate in 1647 and was crowned King of Poland in 1649.
An in pectore cardinal is a secret appointment known only to the appointing pope, typically made to protect the individual or congregation from political persecution. If the pope dies before publicly revealing the name, the appointment lapses entirely. Pope John Paul II was the last pope known to have named in pectore cardinals, including one whose identity was never disclosed.