The Cathars (they did not call themselves that) were a Christian sect that existed in Europe from the 10th to the 15th centuries. They were labeled as heretics for their beliefs by the Catholic Church and were persecuted at times.
The major reason that Cathars were heretics is that they believed in dualism. They believed that there is a good God who made the spiritual universe and an evil entity that made the physical world. They were also heretical because they did not believe in the sacraments of the Catholic Church. They rejected most sacraments and they rejected the authority of priests and bishops. Instead, they advocated for a church based mostly on personal piety and asceticism.