Manichaeism was one of the major ancient religions. It seems mostly extinct today in an organized form. "Neo-Manichaeans" is an attempted revival, but most of the writings of the prophet Mani have been lost. Some scholars and anti-Catholic polemicists believe that its influence subtly continues in Christian thought, through Augustine of Hippo, who converted to Christianity from out of the teachings and practice of Manichaeans.
The religion was founded by Mani, who lived approximately 210-275 A.D. in western Persia. Mani was raised as an schismatic Christian (an Elkasite) but, upon reaching maturity he split with his origins and created a new religion intended to combine all the existing religions. The large existing religions, most notably Christianity and Zoroastrianism, refused to be combined. Mani died in prison awaiting execution by the Persian Emperor. (Alternate accounts have it that he was either flayed to death or beheaded.)
The Manichees made every effort to include all known religious traditions. As a result they preserved many apocryphal Christian works, such as the Acts of Thomas, that would have been lost otherwise. Mani was eager to describe himself as a "disciple of Jesus Christ", but the orthodox church rejected him as a heretic. Mani declared himself, and was also referred to, as the Paraclete (a Biblical title, meaning "helper", which the Orthodox tradition understood as referring to God in the person of the Holy Spirit). The title was later applied to Muhammad.
The Manichaean church spread both east and west. Its most famous western convert was Augustine of Hippo who, after a 8-9 years, changed sides and became an orthodox Christian and a potent adversary against Manichaeism. In the east it spread along the trade routes as far as Changan, the capital of Tang dynasty China.
The Manichee church had no central organization and, if Augustine can be trusted, no appreciable intellectual appeal, at least in the west. In the east it seems to have been destroyed by Islam, and the chaotic period that followed the collapse of the Chinese Tang dynasty.
The most striking principle of Manichee theology is its dualism. The universe is a battlefield for control between an evil material god and a good spiritual god. Christians recognized the evil god in Satan but, of course, could not accept the idea that Satan had as much power as Jehovah, and held that Satan, unlike God, is a created being. The term Manichaeistic is often used to describe any religion with a similar concept of struggle between good and evil.
How much influence the Manichees actually had on Christianity is still being debated. It has been suggested that the Bogomils and the Cathars were only superficially orthodox Christians and were, in essence, Manichees. The record is confused because medieval writers used Manichee as a synonym for heretic. Priscillian and his followers apparently tried to absorb what they thought was the valuable part of Manicheaism into Christianity.
In the case of the Cathars it seems that the Cathars adopted the Manichee principles of church organization but none of its theology.