When entering somebody’s house in India, it is imperative that you remove all footwear.
Using your left hand, especially when eating, is frowned upon, as it is custom to use your left hand when completing sanitary tasks.
To greet or welcome somebody, clasp the palms of both hands together.
Unless the person explicitly shakes their head in disagreement or adamantly replies negatively with some sort of head movement, assume all other head movements to mean that the person is following or agreeing with what you are saying, as many of these head movements can be very ambiguous to the outsider.
Gifts are not expected until a relationship develops. Acceptable gifts include chocolates, flowers that are not white, and items from your home country.
Never refuse food but do not feel obliged to finish your plate. If absolutely necessary, ask for smaller portions.
When you negotiate, start with small talk first. Negotiations tend to be slow and done by the most senior person. Expect a lot of uncertainty while negotiating.

https://www.cia.gov/library/publications/the-world-factbook/geos/in.html (Majority of information)
http://www.censusindia.gov.in/2011-prov-results/paper2/data_files/India2/Table_2_PR_Cities_1Lakh_and_Above.pdf (City population data)
Wolfram|Alpha (Currency exchange data)
Information accurate to best of ability as of 8 November 2016
¹India's census does not take into account ethnicity. The numbers listed are approximations from 2000.