The 4 Varna Groups
The four ranked varna groups were created from the various parts of the body of the primordial man, which Brahma created from clay. They are described in the earliest part of the Rg-Veda: Brahmans, Kshatriyas, Vaishyas, Shudras. The top four ranked varnas are referred to as "twice-born" (dvijas) in reference to the Hindu sacred thread ritual initiation which is suppose to represent re-birth.

These four groups are considered "clean castes." Typical roles of these four varnas are: Brahmans - priests, holy men, arbiters Kshatriyas - kings, warriors, soldiers that protect and guard the country Vaishyas - businessmen, traders, commercial class Shudras - farmers, producers, peasants

The Untouchables
Below the four varna castes lays a fifth group - The Untouchables. This group is not mentioned in the Vedas records until the Bhagavad-Gita and is supposed to be considered outside of the human society and of no caste. Unlike the upper four, it is not considered a "twice-born," having not gone through the Hindu sacred thread ritual initiation. The fifth varna is deemed "unclean" and polluting. Their role in life and society is to perform menial, degrading jobs.
The Untouchables are known by a variety of names. They call themselves "Dalits", priding themselves for the struggles they have fought against society. Mahatma Gandhi calls them "Harijans" - Children of God - when he adopted, against the caste rules, an Untouchable child. Finally, the Untouchables are also known as the "Scheduled Castes" after the 1935 constitution was erected to protect them in society both economically and socially.

"In India there are approximately 240 million Dalits. This means that nearly 25% of the population is Dalit. It also means that in a country, where everybody is supposed to have equal rights and opportunities, 1 out of 5 persons is condemned to be untouchable." (Web Source) Despite the professed equal rights and duties in the Indian society, the Untouchables are still bounded in ways that keeps them in their place. They do not receive any education that can help them climb up in society and they live in poverty with scarce means of amenities, food, health care, and all other key survival needs in life.