The Cathars were a religious group who appeared in Europe in the eleventh century, their origins something of a mystery though there is reason to believe their ideas came from Persia or the Byzantine Empire, by way of the Balkans and Northern Italy.
Records from the Roman Catholic Church mention them under various names and in various places. Catholic theologians debated with themselves for centuries whether Cathars were Christian heretics or whether they were not Christians at all. The question is apparently still open. Roman Catholics still refer to Cathar belief as "the Great Heresy" though the official Catholic position is that Catharism is not Christian at all.
The religion flourished in an area often referred to as the Languedoc, broadly bordered by the Mediterannean Sea, the Pyrenees, and the rivers Garonne, Tarn and Rhône -— and corresponding to the new French region of Occitanie (or the old French regions of Languedoc-Roussillon and Midi-Pyrénées)
As Dualists, Cathars believed in two principles, a good god and his evil adversary (much like God and Satan of mainstream Christianity). The good principle had created everything immaterial (good, permanent, immutable) while the bad principle had created everything material (bad, temporary, perishable). Cathars called themselves simply Christians; their neighbours distinguished them as "Good Christians". The Catholic Church called them Albigenses, or less frequently. Cathars.
Cathars maintained a Church hierarchy and practiced a range of ceremonies, but rejected any idea of priesthood or the use of church buildings. They divided into ordinary believers who led ordinary medieval lives and an inner Elect of Parfaits (men) and Parfaites (women) who led extremely ascetic lives yet still worked for their living - generally in itinerant manual trades like weaving. Cathars believed in reincarnation and refused to eat meat or other animal products. They were strict about biblical injunctions - notably those about living in poverty, not telling lies, not killing and not swearing oaths.
In the Languedoc, known at the time for its high culture, tolerance and liberalism, the Cathar religion took root and gained more and more adherents during the twelfth century. By the early thirteenth century Catharism was probably the majority religion in the area. Many Catholic texts refer to the danger of it replacing Catholisism completely.
Catharism was supported or at least tolerated by the nobility as well as the common people. This was yet another annoyance to the Roman Church which considered the feudal system to be divinely ordained as the Natural Order (Cathars disliked the feudal system because it depended on oath taking).
From 1208, a war of terror was waged against the indigenous population of the Languedoc and their rulers: Raymond VI of Toulouse, Raymond-Roger Trencavel, Raymond Roger of Foix in the first generation and Raymond VII of Toulouse, Raymond Trencavel II, and Roger Bernard II of Foix in the second generation. During this period an estimated half-million Languedoc men, women and children were massacred, Catholics as well as Cathars. The Crusaders killed the locals indiscriminately - in line with the the famous injunction recorded by a Cistercian chronicler as being spoken by his fellow Cistercian, the Abbot in command of the Crusader army at Béziers.
The Counts of Toulouse and their allies were dispossessed and humiliated, and their lands later annexed to France. Educated and tolerant Languedoc rulers were replaced by relative barbarians; Dominic Guzmán (later Saint Dominic) founded the Dominican Order. Within a few years the first papal Inquisition, manned by the Dominicans, was established explicitly to wipe out the last vestiges of resistance.
At the end of the extermination of the Cathars, the Roman Church had proof that a sustained campaign of genocide can work. It also had the precedent of an internal Crusade within Christendom, and the machinery of the first modern police state that could be reconstructed for the Spanish Inquisition, and again for later Inquisitions and genocides. Chateaubriand referred to the crusade as "this abominable episode of our history". Voltaire observed that "there was never anything as unjust as the war against the Albigensians".
Today, there are still many echoes of influences from the Cathar period, from International geopolitics down to popular culture. There are even Cathars alive today, or at least people claiming to be modern Cathars. There are historical tours of Cathar sites and also a flourishing, if largely superficial, Cathar tourist industry in the Languedoc, and especially in the Aude département.