The caste system is a system that divides Hindus into four main categories - Brahmins, Kshatriyas, Vaishyas and the Shudras. Sorta like a pyramid of food groups in health. These castes were based on peoples daily jobs/occupations. Outside of the caste system there were the untouchables or otherwise known as the "the Dalits".
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Is a band of people who terrorize people or peoples as well as follow though with acts of terrorism, such as bombings, suicide bombing, shoot outs and so forth.