Greek Cross: All arms are of equal length.
1 Corinthians 10:18: See jrbell.com. "For the message of the cross is foolishness to those who are perishing, but to us who are being saved it is the power of God." 1 Corinthians 10:18
Andrew's Cross: Tradition says that while Andrew was preaching in Greece he was put to death on a cross of this type.
Crucifix, St. Katherine (Catherine) of Sienna: From a child, Katherine was very religious, living at home in extreme self-mortification, spending much time in prayer and meditation. Later she felt called to leave home and devoted herself to the care of the sick and other good works. Died 1380 A.D.
Fish, Icthus and Cross: A secret sign used by the early persecuted Christians to designate themselves as believers in Jesus. The initial letters for the Greek words for "Jesus Christ, God's Son, Savior", spell the Greek word for fish (see ICTHUS in the glossary).
Celtic Cross, Cross of Iona: Dating back to early centuries of the Christian era, it is said to have been taken from what is now Ireland to the island of Iona by Colomba in the 6th century.
Maltese Cross: Consisting of four spearheads with points together. Dates back to the days of the Crusades when the order of the Hospitallers used it for their emblem. Later they made their headquarters on the island of Malta.
Tau Cross: This form of cross (resembling the Greek letter Tau) predates the Latin cross.
Skull and Crossbones: Occasionally seen at the feet of Christ on a crucifix to symbolize Christ's victory over death or Golgotha (Place of the Skull, the location of Christ's crucifixion).
The three Latin crosses represent the crucifixion location at Calvary (Golgotha). Jesus was crucified between two other convicted criminals.
Cross of Triumph: Symbolizes the triumph of the Gospel throughout the earth.
Anchor Cross: Used by the early Christians in the Catacombs. Ancient Egyptian in origin.
St. Andrew's Cross: The form of cross traditionally associated with the crucifixion of the apostle Andrew, who supposedly requested a form of cross unlike that of his Lord.
I.N.R.I.: Initial letters for Latin inscription on the cross: Jesus Nazarenus Rex Iudaeorum, "Jesus of Nazareth, King of the Jews." Symbol for Jesus.
Crown and Cross: These symbolize the reward of the faithful in the life after death to those who believe in the crucified Savior. "Be thou faithful unto death and I will give thee the Crown of Life", Rev. 2:10.
Celtic cross, St. Columba (St. Colum): This saint founded many churches and monasteries in Ireland and Scotland, the most famous of which was on the island of Iona. One of the most consecrated and indefatigable of Christian missionaries. Died 597 A.D.
Fylfot, Swastika: A pagan symbol adopted by the Christians in the Catacombs and ancient cemeteries because they saw in it a cross concealed from their enemies. This symbol is ancient and has been used by many cultures over many centuries. See this web page
Latin Cross: The most common depiction of a Christian cross in modern times.
Jerusalem Cross, Crusader's Cross: Usually has four small crosses between the arms, the five crosses symbolizing the five wounds of Christ. Worn by Godfrey de Bouillon, first ruler of Jerusalem after the liberation from the Moslems.
Russian Orthodox Cross: BUILD YOUR OWN!
Nikki Johnson adds: The three barred cross is used by many Eastern Orthodox Churches and even by Eastern (Byzantine-rite) Catholic churches. While Slavic in origin, its use extends outside of the Slavic people groups. The common understandings of the bars on the cross among Eastern Orthodox Christians include the following:
1. The top bar is symbolic of the sign placed above Christ's head reading "King of the Jews".
2. The bottom bar, which is sometimes depicted going straight across, is symbolic of two things, at least. When it is slanted, the right side (from the perspective of Christ on the cross looking out) is pointed up, reminding us of the man crucified on the right side of Christ, about whom Christ said, "Today you shall be with me in paradise." The left side points down, reminding us of the demise of the unrepentant man on the left. Also, some say that the slant of the bottom bar represents victory over death - that Christ has broken death's power.
Rope on cross, St. Julia: Julia was a Christian slave girl with a pagan master who respected her faith and goodness. However, when visiting a foreign country the pagan governor ordered her to sacrifice to the gods. Her refusal brought swift crucifixion. Fifth Century.
Christ's cross, St. Helena (Helen): Mother of Constantine the Great and legendary discoverer of the true cross of Christ at Jerusalem. She built the Church of the Nativity at Bethlehem, the oldest Christian church in the world. Died 328 A.D.
Jesus Christ, Conqueror: Consists of Greek cross with abbreviated Greek words for Jesus Christ (abbreviations indicated by horizontal lines), and ΝΙΚΑ. My original source translates this as CONQUEROR, web visitor Pfarrerin translates this as VICTORY, and Babelfish translates it as OVERCOME.
Latin Cross, Five Pointed Star, Heart: A combination of a five-pointed start, a heart and a Latin cross. Search for STAR, HEART and CROSS to see related symbols. Thanks to Lezann.
Cross on the Rock: The rock is a symbol of our Lord, based on I Cor. 10:4. With a cross, it suggests the words of the Venite Exultemus, "Let us make a joyful noise to the rock of our salvation."
Three-Step Cross, Graded Cross: The steps, from the top down, stand for Faith, Hope and Charity.