The cardinal is, above all, an elector: the office exists chiefly so that there is a defined body of men who choose the next pope.
The conclave
The central and defining function of the cardinals is to elect the Bishop of Rome, which they do gathered in conclave on the death or resignation of a pope, and since the twentieth century the right to vote has been limited to those under the age of eighty. Around this function the cardinals also serve as the chief counsellors of the pope and hold the most senior administrative offices of the church, so the college is at once the electorate and the senior council of the Catholic Church, its members drawn from bishops around the world.
Appointment and rank
A cardinal is created by the pope, who names those he chooses to the college, so entry to the office is by papal appointment alone, and almost all cardinals are already bishops. The rank is one of dignity and function within the church rather than a grade of holy orders, the cardinalate being an office attached to the Roman church rather than a further ordination. The distinctive scarlet, worn from the cassock to the zucchetto, marks the rank at sight and signifies, by tradition, a readiness to shed blood for the faith. The office has no true parallel outside the Catholic Church, being bound up with the particular institution of the papacy.