OC vs SC vs ST vs BC vs OBC
The caste system in India is considered to be very old, running down since ages. Ancient Hindu society was divided into four exclusive, hereditary, and occupation based Varnas (Castes, or breed, or races). Vedas (ancient Hindu scriptures) which form the basis of such a division of the society into Varnas say that these 4 Varnas originated from 4 different body parts of Lord Brahma, the creator of the Universe. Brahmins originated from the mouth which gives them the right to look after intellectual and spiritual needs of society. Khatriyas (Warriors) originated from hands thus giving them the right to be protectors of the society. Vaishyas (merchants) originated from thighs to look after agriculture and commerce, and the feet gave birth to Shudras (artisans and labor) who were to look after manual work. A fifth category was later added and that was Ati Shudras (Untouchables) who were condemned for all dirty and polluting jobs.
This Varna system worked well till late in the nineteenth century, but as urbanization took place and the economy became more complex, especially after independence in 1947, the Varna system gave rise to the Jati system which had same characteristics as Varna system but Jatis were not subsets of Varnas. There are regional differences in Jati system where a Jati may be backward in a particular region while it may not be so in another region.
To simplify the distinction, and also to provide for up-lifting of the weaker sections of the society, the government of India, with an amendment to the constitution, allowed for reservation of seats for backward and weaker sections of the society. The classification made by the government is as follows.
OC
Other category, also called open category which has no reservation in employment. This is also known as General (GEN) class mainly comprising of three of the highest classes in the Varna system, which are the Brahmins, Kshatriyas and Vaishyas.
ST
These are tribes that have traditionally lived in the forests, making up 7-8% of Indian population. They have traditionally been marginalized and not in the mainstream of the society. They are also known as Adivasis, and are called scheduled tribes as they have been added under a schedule of the constitution.
SC
These are scheduled castes that were in earlier times considered as untouchables comprising 16-17% of the total population of the country.
BC
Also called backward classes, these come from economically and socially backward classes of the society.
OBC
Other backward castes form a very large group that is heterogeneous and similar to ST in the sense that it has been considered by the constitution as being economically and socially very backward. A large chunk (30%) of Indian population belongs to this class.
It was the intention of the policy makers that by providing a reservation for SC and ST in jobs, they would slowly come in the mainstream of the society and this is why this reservation was initially planned for 10 years only. But it has not only continued but even been increased to a great extent causing discontent among the youth of the country.