Rooted in religion and based on a division of labor, the caste system, among other things, dictates the type of occupations a person can pursue and the social interactions that she may have. Castes are an aspect of Hindu religion. Other religions in India do not follow this system.
-istan is a suffix in the Persian language (Farsi), denoting a place. Thus, Pakistan means place of the pure (pak=pure). It does not just refer to geographical areas, as in bimarestan=place of the sick=hospital. The suffix is widespread in Eurasia because of Persian influence (the Mughals of India for example used Farsi).
As one of the most populous countries in the world, Pakistan is also home to some of the world's megacities. Its most populous city, Karachi, is not only the largest in Pakistan but is also the 7th most populous city in the world, boasting a population that exceeds 11 millon people.