Castes were the central feature of people's identities in ancient India. Beginning soon after the Indo-European invasion, about 1500 BC, people in India began to divide everyone into one of five groups, or castes. People thought of the caste system as people's mirror of the way the universe worked. Just as the sun and the planets each had to follow its prescribed path, in the same way people had to live according to their caste. This parallel between the real world and the caste system made caste seem natural and impossible to change or avoid. Caste gave some people special rights and privileges that other people did not have.
Yale professor Mridu Rais talks about caste
Untouchables usually did the worst jobs, like cleaning up people's poop from the gutters, or collecting garbage.
The lowest of the castes was the shudras - the servants and farmhands who did not own their own business or their own land, and who had to work for other people. But gradually a lot of land-owning farmers fell into this caste, too. Probably the largest number of people belonged to this caste.
Above them were the vaishyas, or farmers and traders, who owned their own farms or businesses. There were a lot of them, too.
Above these were the Kshatriya, or warriors. There were not very many Kshatriyas. A lot of Ksatriyas were in the army, or leaders in other ways. Women could be Kshatriyas, whether or not they were actually warriors.
The most powerful caste was the Brahmans, the priests and leaders. There were only a few of them. Only Brahman men were allowed to go to school, or to teach in schools (Brahman women could not go to school).
There were also a lot of smaller groups within these castes. People who came from different castes could not eat together. Usually people from one caste did not marry or make friends with people from another caste.