The Herodians were a Jewish political or court party that supported the Herodian dynasty and its policy of cooperation with Rome. They were not a religious sect. Their political interests included preserving a measure of Jewish government under the Herods and submitting to Roman authority.
The Herodians joined the Pharisees in opposing Jesus. They participated in the attempt to trap Him with the question of tribute money and were also associated with the plot against Him after the healing of the man with the withered hand.
The party favored the Herodian rulers, who held their territories by Roman grant, and therefore supported paying tribute to Rome. Jesus’ warning about the “leaven of Herod” may have referred to the party’s insidious spirit, though the supplied material presents this as a possibility.