First Degree Price Discrimination - The monopolist sells different units of the commodity at different prices which differ from person to person.
Second Degree Price Discrimination - The monopolist sells different units of the commodity at different prices. The distinction from the first case is that the people buying the same amount of the commodity pay the same price. Bulk discounts are an example of this practice.
Third Degree Price Discrimination - In this case different prices are charged to different people. But for each person each unit of the commodity costs the same. An example of this is the discounts given to students or senior citizens.