Four Horsemen of the Apocalypse

a painting by Albrecht Dürer

The Four Horsemen of the Apocalypse appear in the Christian Bible in chapter six of the Book of Revelation. The verses traditionally describe the four horsemen as Conquest, War, Famine, and Death.

White Horse:

"...When the Lamb had opened the first of the seven seals, I heard the first of the four beasts say with a thundering voice, Come and see. And I beheld, and lo a white horse; and he that sat on him had a bow: and there was given unto him a crown, and he departed as conqueror and to conquest." Revelation 6:1 - 6:2

The white horse of the apocalyptic four may represent polar qualities of evil or righteousness, depending on interpretation.
Some interpret the rider of the white horse to be the anti-christ, or a representation of false prophets, citing differences between the white horse in Revelation 6 and Jesus on the white Horse in Revelation 19.
The rider of the white horse may represent the second coming of Christ during which the end of the world/judgment day would occur. He may also just represent judgment of the living and the damned. Besides Christ, the horseman could represent the Holy Spirit.

Red Horse:

"And there went out another horse that was red: and power was given to him that sat thereon to take peace from the earth, and that they should kill one another: and there was given unto him a great sword." Revelation 6:4

The rider of the second horse is generally held to represent War. The red color of his horse represents blood spilled on the battlefield. He carries a greatsword, which represents battle and fighting.

Black Horse:

“And when he had opened the third seal, I heard the third beast say, Come and see. And I beheld, and lo a black horse; and he that sat on him had a pair of balances in his hand. And I heard a voice in the midst of the four beasts say, A measure of wheat for a penny, and a quart of barley for a penny; and see thou hurt not the oil and the wine.” Revelation 6:5 - 6:6

The third horseman, riding the black horse, is called Famine. The black color of the third horse could be a symbol of the dead.

Pale Green Horse:

“And when he had opened the fourth seal, I heard the fourth beast say, Come and see. And I beheld, and lo a pale green horse; and he that sat on him was called Death, and Hades followed with him. And power was given to them over a fourth of the earth, and that they (the four horsemen) should kill with sword, and with hunger, and with death, and with the beasts of the earth.” Revelation 6:7 - 6:8

The fourth horseman, usually said to ride on a pale horse, is explicitly named Death. Of all the riders, he is the only one whose name is easily distinguishable. Generally, he is usually followed by Hades. The greenish color of the fourth horse could possibly represent plague, fear, sickness, and/or decay.