Cardinals are not a part of the three-fold sacramental character of Holy Orders. In the Sacrament of Ordination, a man is ordained to the deaconate, presbyterate, and episcopate, in other words deacon, priest, or bishop. The term cardinal is an honorary one conferred by the pope, usually to a bishop.
The word cardinal comes from the Latin word cardo, which means hinge. Cardinals under the age of eighty years have the great privilege of electing a pope. Together, the cardinals form a group known as the College of Cardinals or Sacred College, which is an important advisory board to the pope; they have the privileged confidence of the pope. It is in this sense that they are the hinge for the pope to govern the church.
Cardinals have worn red since the time of Pope Innocent IV in 1245. The red symbolizes that they are ready to lay down their life for Christ and His Gospel. They are referred to as Princes of the Church because of their ecclesiastical equivalency to a prince in a secular kingdom.
Cardinals can be heads of important archdioceses throughout the world. They are the people in command of important offices in the church, such as congregations and dicasteries. A cardinal may even be the Primate of a country, the archbishop who has authority over a national territory rather than just his own diocese or province. Usually, it is because the Primate’s diocese was the first one to be founded in that country, hence it is called the Primatial See (another word for diocese). The Primate may also convoke and preside over national councils.
Most cardinals are also ordained bishops. There are a few that are only priests. Canonically speaking, laymen can also be given the title of cardinal by the pope. There are three levels of cardinals. These levels, however, do not coincide with their ordination, although they use the same names. There are terms like cardinal bishop, cardinal priest, and cardinal deacon, even though they are all ordained to the episcopacy. These are honor levels of distinction, much akin to monsignors.
Prothonotary apostolic is the highest monsignor, domestic prelate, the middle, and papal chamberlain is the lowest.