Price discrimination It is a situation when a monopolist charges different prices from different buyers of the same product. This is generally done to maximise profits, e.g. in a monopoly, surgeon in your area may charge low fee from the poor patients and high fee from the rich patients.
Product differentiation It is a situation when different producers under monopolistic competition try to differentiate their product in terms of its shape, size, packing, trademark or brand name. This is done to attract buyers from the rival firms in the market.