Monopolistic competition
Many buyers and sellers, trading over a range or prices, caused by differentiation
Either the physical product can be varied in quality, features, style; or the accompanying services can be varied => buyers see a difference in sellers’ products and are willing to pay different prices for them
E.g. foods
Oligopolistic competition
A few sellers who are highly sensitive to each other's pricing and marketing strategies; product can be uniform (steel, aluminum) or non-uniform (cars, computers)
Difficult for new sellers to enter the market
Seller interactions are game-theoretically complex