I am responding because this is an important question and many people are interested in it. The short answer is whether you do any type of business that involves “making money out of the money”, such as lending, investing and selling stocks comes from the NBFC’s purview. Even if you do it with your own money and don’t take public money to do it (investing/trading), you still have to register as an NBFC with RBI which is not an easy process. You must have a main business as above and a minimum capital investment of Rs 2 crore.
This is what I learned through the RBI website and other online sources. Perhaps a better educated CA can help you the most.