The two main categories were the convention states who had agreements with British India regarding mail, and the feudatory states who ran their own posts, and whose stamps were only valid within their borders.
The convention states all used contemporaneous stamps of India, overprinted with the name of the state, in Latin letters or Hindu letters or both, depending on state and period. The convention states were:
The stamps of the convention states all became invalid 1 January 1950.India had a great many feudatory states, but not all issued stamps. In some states the stamps served a legitimate purpose, but in others the stamps were issued more to please the ruler's vanity, and normal mail was franked with British India's stamps. The feudatory states issuing stamps were (the dates are the starting and ending dates of stamp issuance):