India's caste system, which splits up Hindus into different societal groups according to their work and birth, is thought by researchers to go back some 3,000 years.

In the system, Hindus are divided up into four classes based on the principle of "varna," which literally means "color": the Brahmins (the priestly class); the Kshatriyas (the ruling, administrative and warrior class); the Vaishyas (the class of artisans, tradesmen, farmers and merchants); and the Shudras (manual workers). There are also people who fall outside the system, including tribal people and the Dalits, previously known as "untouchables," although the term is somewhat contentious.

The concept of "jati," meaning "birth," also underlies the caste system and causes its differentiation into thousands of sub-groups based on lineage or kinship that are difficult to define.

The system has led to the upper castes being privileged over the lower castes, which were often repressed by those higher up on the caste scale. For centuries, inter-caste marriage was forbidden, and in villages, castes mostly lived separately and did not share amenities such as wells.

Read more: Indian man hacked to death over inter-caste marriage

Moving toward change

The caste system was bolstered under the British Raj, which appointed only upper-caste Hindus to senior positions and administrative jobs. During the 1920s, however, protests led to the colonial administration introducing a system of quotas under which a certain percentage of government jobs were reserved for lower-caste Hindus.

Read more: Caste protests spotlight India's contentious quota system

After India attained independence in 1947, the country introduced laws to make discrimination against lower castes illegal and to improve their socioeconomic positions. Quotas were introduced for college admissions and jobs.

As a result, some Dalits have made it to leading positions, such as BR Ambedkar, who wrote the Indian constitution, and KR Narayanan, who was elected president in 1997.

Read more: Low-caste Dalits face off in Indian presidential contest

Inequalties under the system still exist in modern India despite these measures, which have even served to reinforce the divisions to some extent. Violence based on caste has also erupted in recent times, much of it involving attacks on Dalits.

The caste system has also spilled over into other religions in India, with Christians, Muslims, Sikhs and Jains all employing similar forms of social stratification.