Also known as Albigensians
It is believed the term Cathar comes from the Greek word Katheroi, meaning pure. The name Albigensian resulted from the preaching efforts of the Catholic Church in Albi, which was no more sympathetic to the sect than any other towns at the time. Albi likely became associated with the Cathars due to a preaching mission taken there by St. Bernard of Clairvaux. The popularity of the saint, combined with his efforts in the town, likely provided the widespread, but innaccurate, association of Albi with Catharism. The Catholic Church sent numerous preaching missions to the Cathars, not only including the likes of St. Bernard, but also St. Dominic in the thirteenth-century.
The Cathars were deemed heretical by the Catholic Church, which claimed they were a type of neo-manichean dualists, believing in two gods, one of the spiritual world and one of the physical world. According to Inquisitors records, the Cathars believed the spiritual world was evil and, as a result, advocated vegetarianism and non-reproductive sex. The ultimate goal was the liberation of the spirit from the prison of the flesh.
Additionally, they reportedly believed in reincarnation, rejected the Old Testament, rejected the sacraments of the Catholic Church, rejected material possessions, and rejected the image of cross. They also had a clergy known as the "perfect", which were distinguished from other Cathars, known as "good Christians", by black clothing. The perfects abstained from meat and marriage, for which the general population of Cathars showed them reverence and respect. Although ascetic doctrines were advocated and adhered to by the perfects, other Cathars continued to own material possessions, have families, and likely ate meat. They believed in only one sacrament, known as the Consolamentum, in which a perfect would lay his hands on either a dying believer or one who wished to adopt the ascetic lifestyle of the perfects. Supposedly, after receiving the sacrament, the perfects would no longer fear death.
Interestingly, some modern protestants have viewed the Albigensians as a type of forerunner of the protestant movement. This is because of the Albigensian resistance to Catholic authority, but they seem to be unaware of some of the more unusual doctrines of the Cathars. See for example, the notoriously a-historical but popular work Trail of Blood, authored by J.M. Carroll in 1931. Any search of the web will show the work is still promoted and sold by some protestant Churches into the twenty-first century.
The first Cathar Bishopric was establised in France in 1150, and by the end of the twelfth-century they had at least eleven bishoprics in France and Italy. They were clearly growing in number and influence throughout the region, and the Catholic Church found itself increasingly concerned. The Church expanded its activity in the region during Pope Innocent III's reign, finally resulting in the murder of the Papal Legate Peter Castelnau in 1208. The incident has been cited as the spark for Innocent III's calling of what became known as the Albigensian Crusade.
The nobility of northern France was willing to make war against the wealthy nobility of the south of France, and the crusade continued with varying degrees of intensity for over twenty years, finally ending in the Treaty of Paris in 1229. Then the Inquisition moved into the region and began the lengthy process of extirpating the heresy, with occasional outbursts of violence in the storming of various Cathar strongholds fortified by castles. The last known Cathar perfect died in the beginning of the fourteenth-century.
One can still travel to the south of France and visit the remains of numerous Cathar castles including Queribus and Montsegur. They have become an integral part of the modern tourism industry of the region and are well worth the trip for a student of the Albigensian Crusade or the Cathars.
An interesting side note on the Albigensian Crusade is that it provides the basis for the well known and popular saying among U.S. members of the military, "Kill'em all and let God sort'em out." This saying is printed on T-shirts and heard on U.S. military bases as an informal creed for more intense military groups like the Marines, Rangers, and Special Forces. It was initially Arnaud Armaury, Abbot of Citeaux, who when asked how to distinguish which citizens of Beziers were Catholics and which were Cathars, reportedly replied, "Kill them all, God will know his own." He may have never have actually said it, as it comes from a single source written decades after the event.