Cyril and Methodius. Immaculate Heart of Mary, Cleveland, O.
Many like stained glass windows. Windows bring in light. In part, stained glass mimics a prism. Stained glass adds to the ambiance of sacred space. Stained glass became associated with gothic architecture, early modern Europe, not much. Then a revival came with 19th century romanticism. A painterly style was very successful in south Germany, and very well distributed throughout the world.
Some churches do not have colored glass. Some Catholic churches do not have colored glass, though in this area not many. There is some overlap of subjects between Catholic and Protestant churches. The tendency is for the Catholic universe to be larger and more numerous.
There are different types of glass. The oldest is mouth blown glass. The glob of molten sand has metallic salts in it, producing the colors. The glass is cut and fixed between leaden cames, almost a mosaic (and older art form). Now there is machine glass. There is painted glass that is fired in a kiln.
faceted glass of Jesus and the Eucharist with loaves and fishes. St Louis, Cleveland Hts
A different sort is faceted glass, dalle de verre. Whereas traditional glass is ¼′′ or ⅛′′ thick, faceted glass is 1′′ thick hammer chipped dalles (slabs of glass akin to flagstone pavers). The areas between dalles are filled with opaque epoxy resin (usually black). Now such dalles can be sorted into a figure. Often they are not, and the epoxy is wide and the result is often bands of color. Sometimes thick bands of metal separate the dalles and epoxy. There are more abstract configurations than with older glass. Overall, there are more aesthetic failures. Some could be windows in storage rooms of commercial buildings, and no one would think they were ever installed in a church or chapel. Some good installations take a time to warm to.
The tendency is for Protestants to have less visual art. Some churches have only colored glass, solid sheets, or panes, or leaded cames with one color; sometimes alternating diamond patterns of two colors, and other geometric forms. An american innovation was opalescent (milky colored, semi-opaque) glass. Of course other religions, or non-religious groups have these also.
Then there are windows that have one symbol surrounded by other panes of colored glass. In christian iconography there is a selection, which includes: butterfly, dove, peace dove, cross, pillar, the veronica, turtle doves, ten commandments twin slabs, cross and crown, chalice, grapes, wheat, bread, fish, thurible, pierced heart, sacred heart, peacocks, pelicans, lily, pomegranates, scallop, alpha, omega, chi rho, harp, anchor, lamb, bells, and more.
Then there are people and scenes. Jesus, Mary and Joseph show up; Jesus most of all, the other members of the Holy Family rarely in Protestant windows outside of nativity scenes.
Many churches have the four evangelists, and or their symbols: Matthew (man/angel), Mark (lion), Luke (ox), and John (eagle). Slightly bigger churches may have the twelve apostles, often with Paul, sometimes Matthias (Judas is rare outside the Last Supper). Some have the four latin doctors (Augustine, Ambrose, Jerome, Gregory the Great). The three greek doctors (John Chrysostom, Basil, Gregory Nazianzen), more so in Byzantine, and Eastern Orthodox. The western church would add Athanasius for a fourth.
Now, Old Testament figures are less common; though here they are maybe more common in Protestant churches that have people depicted. Often four prophets, instead of four doctors, are paired with the gospel writers. The major prophets: Isaias, Jeremias, Ezechiel and Daniel or Moses and Elias (the last two are in Transfiguration windows). Jonas (and the whale), Osee (Hosea), Amos and Michæas are rarer. Zacharias, Abdias, Habakuk, and the rest are rarer still.
Mother of Our Savior. Saint Mel, Cleveland, O.
Catholic churches are filled with Marian Windows. Sometimes many with symbols from the titles in the Litany of Loreto (e.g. Mother Most Chaste, Virgin Most Merciful, Seat of Wisdom, Tower of Ivory, Queen of Martyrs, etc.). Foremost, the scenes of the Rosary, and people do not notice. Now, many protestants have a bias against Mary. Mary always points to Jesus. Many of the mysteries of the rosary are scenes of Jesus, so people who are not familiar with the rosary do not recognise the connexion. Many churches will have one side of the nave lined with the Joyful Mysteries, and the other with the Glorious Mysteries, along with other windows, depending on the number of bays.
Now, there were three sets of five mysteries.
Five Joyful Mysteries
Annunciation, Visitation, Nativity, Presentation, Finding Jesus in the Temple
Five Sorrowful Mysteries
Agony (Gethsemane), Scourging, Crowning with Thorns, Carrying of the Cross, Crucifixion
Five Glorious Mysteries
Resurrection, Ascension, Pentecost, Assumption, Coronation
The late pope, John Paul II, added a fourth. The windows were already there.
Five Luminous Mysteries
Jesus' Baptism, Cana, Proclamation of the Kingdom (this can be any or all of the following: Calling Apostles, Sermon on the Mount, Friend of Children and Men, Giving Peter the Keys, Curing the Sick, Raising the Dead, Forgiving the Penitent, Mary Magdalene Anointing Jesus' Feet), Transfiguration, Last Supper
Often ethnic churches have national saints or scenes of their historical christianity. A church will often have their patron saint on a window. Now, since most Protestant churches don't have a patron saint this is less so, although they sometimes have a memorial to a church member.
St. Francis of Assisi. Saint Cecilia, Cleveland, O.
Many churches have a window of Saint Cecilia (the patron of music) in the choir loft. Other popular saints are Anthony of Padua, and Thérèse of Lisieux. Windows of the parables are made, the most frequent—the Prodigal Son.
This is not exhaustive. There are so many churches, and so many opportunities. Some are thematic, like the seven sacraments. At Francis de Sales, Parma there are the first five popes on one side of the nave, and the first five of the 20th century on the other; Pius XII was still living when the window was made. Some are unexpected. In the Perpetual Adoration chapel at St. Gregory, South Euclid there is Iscariot with his back turned to you. Judas is always holding the purse, or betraying Jesus.
Saint Stephen, Cleveland has windows of several shapes and of many figures and ornamentation. They had a deluxe order from Munich. They have the common scenes, and the less common ones (death of Joseph, Jesus driving away the merchants from the Temple, Jesus teaching Nicodemus--the John 3.16 window) and they have two large windows of two scenes of the parish patron Stephen, the martyr deacon of Jerusalem, and Lawrence, the martyr deacon of Rome. At Saint Stanislaus, Lorain, there was a small window of God breathing life into Adam. Most people never saw it. It was on the stairwell going to the organ. Last time i saw it, it was misidentified on Lennon's/Henniger's internet trading site.