The Buddhist concept of the devil is called Mara, the head of the heavenly demons and the Sense Desire realm. Buddhists believe that Siddhartha (later, the Buddha) was tempted by Mara before enlightenment, but he could not be swayed from his path. Mara symbolizes desire and everything that hinders humans from proceeding along the right path.
I believe the devil:
Christianity teaches that the devil (or Lucifer/Satan) is a fallen angel, after Isaiah 14:12: "How you are fallen from heaven, O Day Star, son of Dawn!" Lucifer was an angel who rebelled against God and was ejected from heaven.
Catholics believe that the devil and other demons were angels created by God but became demonic--or adversaries of God--after their fall. Satan, lord of demons, exists and can cause humans harm, but he is still a creature and not equal to God.
Though Hindus believe there is evil in the world, there is no single devil-like entity in Hinduism. However, there is a concept of asura, or evil sprit. Evil spirits do not remain evil forever--they are beings of Naraka, the lower plane, and can evolve to goodness. The Bhagavad Gita, one of Hinduism's most important texts, tells the story of Arjuna's fight against evil and lower desires, embodied by the army of Kauravas.
Iblis, the devil in Islam, is described in the Qur'an as "the adversary." Originally, he refused to obey God's commandment to prostrate himself before Adam. The devil in Islam tempts humans and tries to mislead them. Before beginning to read the Qur'an, Muslims recite the Ta'awud, in which they say, "I take refuge in God from Satan the stoned one"--praying that they may take refuge in Allah from the devil.
I believe the devil:
Judaism teaches that humans were created with two inclinations, the yetzer tov, or good inclination, and the yetzer ra, the bad inclination. The inclination to do bad or be selfish is within a person, it is not the result of an outside force. The Jewish concept of Satan, the "hinderer," is that he is an angel who leads humans to evil, and people must struggle to overcome their evil inclinations.
Wiccans do not have a concept of the devil in their beliefs or practices. Wiccans do not believe that good and evil come from a divine source, but instead individual humans are responsible for their actions.
One of the earliest concepts of the devil comes from Zoroastrianism. Zoroaster, founder of Zoroastrianism, taught belief in two distinct entities: Angra Mainyu and Ahura Mazda. Mainyu was an evil spirit who tempted Zoroaster, but Zoroaster overcame evil in favor of Ahura Mazda, the supreme Zoroastrian god and "Wise Lord."