what role did the caste system play in ancient indian society

For centuries, caste has dictated almost every aspect of Hindu religious and social life, with each group occupying a specific place in this complex hierarchy.
In recent years, there have been demands from several communities to be recognised as OBCs – in 2016 there were violent protests by the Jat community in Haryana and the Patel community led huge protests in Gujarat in 2015 demanding access to caste quotas.

The castes were a form of social stratification in Aryan India characterized by the hereditary transmission of lifestyle, occupation, ritual status, and social status. These social distinctions may have been more fluid in ancient Aryan civilizations than in modern India, where castes still exist but sociologists are observing inter-caste marriages and interactions becoming more fluid and less rigid.
Society during the Vedic Period (c.1750-500 BCE) was patriarchal and patrilineal, meaning to trace ancestral heritage through the male line. Marriage and childbearing were especially important to maintain male lineage. The institution of marriage was important, and different types of marriages—monogamy, polygyny and polyandry—are mentioned in the Rig Veda. All priests, warriors, and tribal chiefs were men, and descent was always through the male line.

The subsequent rise of Islam, Christianity, and other religions also left their mark on the original Varna system in India. Converted generations reformed their notion of Hinduism in ways that were compatible with the conditions of those times. The rise of Buddhism, too, left its significant footprint on the Varna system’s legitimate continuance in renewed conditions of life. Thus, soulful adherence to Varna duties from the peak of Vedic period eventually diminished to subjective makeshift adherence, owing partly to the discomfort in practising Varna duties and partly to external influence.
Vaishya women, too, supported their husbands in business, cattle rearing, and agriculture, and shared the burden of work. They were equally free to choose a spouse of their choice from the four Varnas, albeit selecting a Shudra was earnestly resisted. Vaishya women enjoyed protection under the law, and remarriage was undoubtedly normal, just as in the other three Varnas. A Vaishya woman had equal rights over ancestral properties in case of the untimely death of her husband, and she would be equally liable for the upbringing of her children with support from her husband.

Bottom Caste: The Sudras: The Sudras were the unskilled workers. They might find a job on a farm or a non-skilled job in a home or business like cleaning. They were the bottom caste.
For Kids: The idea of the caste system in ancient India began after the Indus Valley Civilization disappeared and a new people arrived, the Aryans. The Aryans were nomads from the north. They brought with them some new ideas. One of those ideas was Hinduism, which is now a world religion. Another was the idea of a caste system. A caste is a life-long social group into which a Hindu is born.

The caste system, as it actually works in India is called jati. The term jati appears in almost all Indian languages and is related to the idea of lineage or kinship group. There are perhaps more than 3000 jatis in India and there is no one all-Indian system of ranking them in order of status. Yet in each local area jati ranking exists and is very much related to purity and pollution. Each jati has some unique job, but not everyone in the jati performs it. Thus there are barbers who do not shave, carpenters who do not build, and Brahmins who do not act as priests. A jati is identified in a local setting by whom its members will accept food and water from and to which jatis its members will give food and water. People will try to marry their sons and daughters to members of their same jati and will give their major loyalty to their jati. A jati will usually be organized into a biradari (a brotherhood), and this organization carries out the business and oversees the working of the jati and has the power to exclude an offender from the jati.
The jati system is not static in which all groups stay in the same position. There is mobility in the system and jatis have changed their position over the centuries of Indian history. However, the jati moves up the social scale as a group and not as individuals. A jati can improve its position in the class system by advancing economically and emulating social groups with money and power. At the same time, a jati can also move up in the caste hierarchy. Mobility in the caste system has been termed “Sanskritization” by the scholar M.N. Srinivas. To gain position in this process, a lower jati copies the habits and behavior patterns of the dominant jati in the area. This may mean a lower jati will change its name to one of a higher jati, adopt vegetarianism, observe more orthodox religious practices, build a temple, and treat its women in a more conservative way. The type of emulation will depend on the habits of the dominant jati being copied. If the jati can gain acceptance for its new name, new history, and new status, it will then marry its daughters to members of the jati in which it is seeking to gain membership. In due time the new position on the social scale will be solidified and accepted by other jatis. This practice is not totally unlike that of immigrant groups coming to America and copying the habits of the WASPs who were in control. In your own community you could probably identify the most prestigious group of people and observe other members of the community copying their behavior in ways such as sending their children to dancing classes and summer camps, and putting braces on their teeth.

References:

http://courses.lumenlearning.com/suny-hccc-worldcivilization/chapter/the-caste-system/
http://www.ancient.eu/article/1152/caste-system-in-ancient-india/
http://india.mrdonn.org/castesystem.html
http://asiasociety.org/education/jati-caste-system-india
http://www.princetonreview.com/college-advice/application-essay-topics