"Spiritual warfare" is the concept in some Christian sects concerning confronting facing evil supernatural forces, such as demons. Prayer is one way Christians practice spiritual warfare, but exorcisms, fasts and using anointing oils are also done.
At its core, spiritual warfare involves using God's power to fight evil spirits. The book of Ephesians in the Bible describes an analogy likening doctrines of faith and spirituality to a suit of armor: a belt of truth, a breastplate of righteousness, a gospel of peace, a shield of faith and a helmet of salvation. Those who believe in spiritual warfare hold that, to have power over evil spirits, a Christian has to have a strong relationship with God. Otherwise, he can't use God's power against evil.
A common belief among many Christian sects is that Satan has lost his prior battle with God in "the war in Heaven," but wants to bring as many people down with him as possible, primarily through lies and deceit.
Although spiritual warfare is most commonly practiced by Christians, it is also performed by those of other religions. It has bee the source of some controversy, however, as some faiths have used it to rationalize the persecution of others who believe differently than they do.