The Bharatiya Jana Sangh (or Jana Sangh) was a Hindu Nationalist political party founded in 1951 as a front organisation of the RSS, incorporating elements of the Hindu Mahasabha. Its creation followed that of the ABVP, and preceded that of a number of other Sangh Parivar organisations.
Important pillars of its ideology were the "integral humanism" of Deendayal Upadhyaya, a conservative vision involving maintenance of the caste system (different castes being like different parts of one body); and the "Indianization" of minorities proposed by Balraj Madhok: minorities should pay allegiance to Hindu symbols and adopt Hindu cultural habits and customs.
In 1977 the Jana Sangh merged with the Janata Party, and re-formed as the BJP after a split in 1980.
References
• Hindu Nationalism: A Reader Edited by Christophe Jaffrelot, Princeton, 2007.