A Kar Sevak is someone who offers services for free (that is, a volunteer) to a religious cause. It originates from the Sanskrit words kar (hand) and sevak (servant). The kar sevaks organized by the Vishva Hindu Parishad for the construction of the Ram temple in Ayodhya were responsible for the demolition of the Babri Masjid.
References
Kar Sevak Wikipedia(Text) CC BY-SA