Perfect competition: 1000 of providers of same good, firms are price takers, horizontal demand curves, and earn zero economic profits due to
Pure Monopoly: Single seller selling unique product, can earn economic profits due its price making power, 100% entry barrier.
Oligopoly: Only a few firms offer similar or identical product. Have price setting power & enter barriers are high thus forming cartels (acting in unison)
Monopolistic Competition: Large # of sellers but sell products that are not exactly the same. Entry barriers exist but are low and can determine pricing up to certain extent. Economic profits cannot be earned in the long run due to competitors & low barrier costs. ( Mex Rest) ATC = MC