Untouchables are the very bottom of the Indian caste system. They were people who, under the system, literally are not be touched because they are considered to be so undesirable. Good karma enables one to improve his caste in the next life (see reincarnation). Today, caste discrimination is illegal in India but still persists in rural areas.
Untouchables are also found in some other parts of the world, such as Gypsies in Europe, Burakumin in Japan, among others.