What is the main criteria used …

The main criterion used by the World Bank in classifying different countries:
Countries with per capita income of US$ 12616 per annum and above in 2012, are called rich countries and those with per capita income of US$ 1035 or less are called low-income countries. India comes in the category of low middle income countries because its per capita income in 2012 was just US$ 1530 per annum. The rich countries, excluding countries of Middle East and certain other small countries, are generally called developed countries.
Limitations :
Limitations of this criterion are that while average income is useful for competition, it does not tell us how this income is distributed among people. A country may have more equitable distribution. People may be neither very rich nor extremely poor. But in another country with same average income, one person may be extremely rich, while others may be very poor. So, the method of average income does not give correct picture of a country.