Who are the "Doctors of the Church"?

Most of us know there are Doctors of the Church, and also Church Fathers. But we tend to confuse them. Of course the Doctors didn't practice medicine (although Albert the Great, doctor of science, probably could have) anymore than the Fathers necessarily had children (although celibacy wasn't enforced universally in the period of the Fathers, roughly encompassing the first seven centuries of the church).

Doctors of the Church are always canonized saints. They're also defined by three criteria: eminent learning, a high degree of sanctity, and proclamation by a pope or council. Their naming is not an infallible decision and doesn't presume their writings are totally free from error. No martyrs are on the list, because a Doctor's primary significance is as a confessor of the faith. This is why martyrs like Ignatius, Irenaeus, and Cyprian (shoo-ins otherwise) didn't make the cut.

Presently there are 33 Doctors, although the number held at eight for many centuries. Originally four Doctors were celebrated in the Western church: Gregory the Great, Ambrose, Augustine, and Jerome. Three were revered in the East—John Chrysostom, Basil, and Gregory Nazianzus—with Athanasius added later to balance the account between West and East. These two groupings were often depicted in church art, holding books to illustrate their teaching role.

In the 16th century a Dominican pope made Thomas Aquinas, also a Dominican, a new Doctor. The next pope, a Franciscan, retaliated by giving the title to Bonaventure, a fellow Franciscan. This reopened the category for nominees so that four were added in the 18th century, nine in the 19th, and ten more in the 20th, including three women since 1970: Teresa of Avila, Catherine of Siena, and Thérèse of Lisieux.

For the record, to date there are five Benedictines; three each of Carmelites, Dominicans, and Franciscans; two Jesuits; and one Redemptorist among the Doctors. The importance of the Doctors' office has not changed in 20 centuries. Each reveals, through his or her unique achievements in apologetics and homiletics, history and education, science and art, music and poetry, catechesis and mysticism, morality and spirituality, another avenue of grace for the whole church.

Scripture
Concerning the church's teaching office: 1 Timothy 1:3-7; 4:6-16; 6:2b-10; 2 Timothy 3:16-17

Online resource
The 33 Doctors of the Church

Books
The Doctors of the Church: Thirty-Three Men and Women Who Shaped Christianity by Bernard McGinn (Crossroad, 1999)
Also see individual Doctors in the Classics of Western Spirituality series by Paulist Press