From Wikipedia (with some editing):
Catharism was a name given to a Christian religious sect with dualistic and gnostic elements that appeared in the Languedoc region of France and other parts of Europe in the 11th century and flourished in the 12th and 13th centuries.
A basic belief was the complete incompatibility of love and power. As matter was seen as a manifestation of power, it was also incompatible with love.
The Church called for a crusade, which was carried out by knights from northern France and Germany and was known as the Albigensian Crusade. The anti-Cathar Albigensian Crusade, and the inquisition which followed it, entirely eradicated the Cathars.
The Cathar theology was actually more reasonable than that of Roman Catholicism – which only increased their desire to eliminate it.
What this article does not say is that the property of the Cathars, by order of the Pope, became the property of those who subjugated them. This had been the policy of all the Crusades, which were often led by noblemen of no property, due to the inheritance laws.