Caste is a closed system of stratification where social groups are divided on the basis of birth and their occupation. Social groups follow the code of behaviour prescribed by tradition regarding marriage and kinship alliances. Caste groups (varnas and jatis) are unequal, ranked on a scale of hierarchy based on their ritual status, from ‘pure’ to ‘impure’. Their status determines with whom they can interact and with whom they cannot. The idea and practice of untouchability is an integral part of the caste system. Untouchability now stands banned in India.

G.S. Ghurye identified six core features of the Hindu Caste system.

Segmental division of society: The caste system has four varnas (Brahamanas, Kshatriyas, Vaishyas and Shudras). Some sociologists argue that Ati-Shudras should be separated from Shudras and they should be counted as the fifth varna. Every varna has many jatis. The jatis are determined by birth and not by choice. The status of a person depends not on the amount of wealth (s)he possesses but on the rank that his or her jati enjoys in the Hindu society. For example, the Aggarwals, a jati of the Vaishyas varna who are traders and merchants may possess more wealth than what the Gaur Brahmins do. But the Brahmins are ranked at the top and Vaishyas are ranked third in the Varna system. Likewise, even when they are not as well-off as the Aggarwals, the Gaur Brahmins will be ranked above than the Aggarwals.
Hierarchy: The four (or five) major caste groups that are called varnas, as well as the divisions within each varna, called jatis, are arranged one above the other. All the caste groups are hierarchically layered in which the most favoured are at the top and the less favoured near the bottom.
Restricted choice of occupation: Generally, each caste considers a particular occupation as legitimate. For instance, Brahmins were meant to be the priests and religious functionaries, ‘specialists’ with regard to rituals, scholars and religious commentators/exegesis. Kshatriyas were meant to be rulers, warriors and administrators. Vaishyas were meant to be traders, merchants and business persons. Shudras were labourers in agriculture, or elsewhere, engaged in manual and menial work. They work as servants to the top three varnas. Ati-Shudras are engaged particularly in menial work, highly polluting occupations such as work involving leather, excreta, or the disposal of dead bodies.
To change the hereditary occupation in pursuit of other work, even though it may be lucrative, was not considered right, not by tradition at least. But today the circumstances have changed. There is still symmetry between caste and occupation/class but there are also asymmetries between these categories. The jatis of the Khshatriya varna are not necessarily the rulers or administrators in a democracy. Anyone can be a ruler. For example, Mr. Narendra Modi, the Prime Minister of India, belongs to an OBC jati. The modern capitalist economy breaks the rigid caste system in terms of choice of occupation. There are people who belong to the OBC jatis or the SC jatis but are very rich, such as Rajesh Saraiya. He is the first Dalit billionaire of India. So the restriction on the choice of occupations is a feature of the caste system but it has weakened now. Today, it has not remained as strong and rigid as the other features.
Restriction on marriage: The caste system has strict rules about marital institutions. Only endogamy is allowed. Exogamy is disallowed. But sometimes men of upper castes marry women of lower castes. This is called hypergamy.
Restriction on social intercourse: People of one jati are restricted to share cooked food and drink with the people of other jatis. The utensils in which foods are served for the lower jatis are considered to be impure. Purity-pollution norms are strictly followed. Dalits are not allowed to use wells that the upper jatis use. By maintaining purity-pollution norms the upper jatis (Brahmins, Bhumihars, Rajputs, etc.) control over many resources such as land, wells, water, rivers from using those and several other resources. The poem Thakur Ka Kuan by Omprakash Valmiki explains the same thing, how every resource is owned by Thakurs and nothing is left for the lower jatis. Here is the last stanza of the poem:
कुआँ ठाकुर का
पानी ठाकुर का
खेत-खलिहान ठाकुर के
गली-मुहल्ले ठाकुर के
फिर अपना क्या?
गाँव?
शहर?
देश?
The fourth and the fifth features of the caste system are well captured by the Hindi dictum of “Roti-beti ke Sambandh”. They separated two caste groups. People of two caste groups may meet and talk, work together and be friends with each other but may not eat together and marry. In this way the caste groups are separated. The caste system is based on the idea of separation.
Civil and religious privileges and disabilities: Segregation of jatis is the most obvious mark of civil privileges. Dalits and Scheduled Castes are forcibly isolated from the society. The lowest jatis and the upper jatis do not meet or touch each other. Untouchability is practiced. James Freeman’s account of Muli, a dalit from a village in Odisha given below shows how untouchability is practiced:

“The first time I saw him, he was sitting on the dusty road in front of one of the small thatch-roofed tea shops in the village with his glass and saucer placed conspicuously beside him – a silent signal to the shopkeeper that an untouchable wanted to buy some tea. Muli was a gaunt forty-year-old with betel-blakened teeth who wore his long hair swept back.”

In the deep south, in places such as Kerala, “untouchability” was often accompanied with “unapproachability”. Religious privileges and disabilities also existed. Only Brahmins could perform certain sacraments. Shudras and Ati-shudras were not even allowed to read the holy scriptures and epics like the Ramayana and the Mahabharata. Only Brahmins and Kshatriyas are allowed to wear the sacred thread called Janew, because these were considered the “twice-born” castes/dwijiya castes. Vaishyas, Shudras and Ati-shudras are not allowed to wear it.

The six core features of the caste system which are based on the idea of separation, segregation and hierarchy have gone through changes over time and were challenged even in ancient India. However, its nature is such and it is so deeply rooted in the psyche of many Indians that in this modern era also, it changes its forms in different contexts but does not go away. The caste system has proved to be resilient.