Explanation

Perfect competition: many firms, identical products, no barriers to entry, perfect information. Price = marginal cost. Rare in real world but useful benchmark.

Monopoly: one firm, unique product, high barriers. Price maker. Can restrict output to raise prices. May require regulation to protect consumers.

Oligopoly: few large firms, interdependent decisions, possible collusion. Examples: telecom, banking. Monopolistic competition: many firms, differentiated products. Examples: restaurants, clothing.