What kind of units or entities are castes?
We do not know; neither does anybody for that matter. It is a matter of fact that (a) they are not occupational groups; (b) they are not social classes (that is, there is no correlation between jatis and the ‘means of production’); (c) appear hereditary (though one could obtain certificates that ‘change’ one’s jati legally); (d) some jatis are organized as some kind of legal or semi-legal associations, some jatis are not.
How do we distinguish between castes?
At one level, answer to this question is easy. There is no need to dig deep into some philosophical differences between the beliefs or notions that any two given caste communities share. That is, clarity on the differences in the conception of truth or the problem of qualia of two different castes is not required to identify and distinguish them as two different caste communities. It is sufficient for our purposes to point out that the Lingayats and Kurubas, for example, share different rituals, practices and festivals. This is enough to say that they are different castes. Interestingly, even the apparently ‘silly’ issues like scrubbing floors[1] are sufficient for communities to differentiate between themselves. Probably therefore, communities fight over those issues that look ‘silly’ to an outsider. This means then that a caste community is not dependent on endogamy as a necessary factor for its existence or its individuality, as modern thinkers of caste believe. Similarly, following the ‘teachings’ of a ‘leader’ like Basava is not a constitutive factor for the Lingayat community. (However, these are empirical questions and all further clarifications should await an extended and serious research.)
At another level, an answer to this question is not easy. One can notice some differences between the Lingayats and, say, Kurubas in a given village based on differences in their practices. However, these differences are not common across India. That is, the Lingayats of (say) Andhrapradesh, Maharashtra and Karnataka do not or need not share common practices. What, then, is the relationship between, if any, the members of a same caste in two different places in India? No one has an answer for this question today. We can draw one significant lesson from this: our knowledge about castes is zilch. Nevertheless, we (that is all those modern scholars who talk about caste problem in India) go about making the most profound looking statements about it.
[1] See Madhav M Deshpande’s post here (Accessed 15 March 2011): http://www.columbia.edu/itc/mealac/pritchett/00litlinks/txt_deshpande_letter.html