Monopolistic competition is a market structure with many small businesses that produce differentiated products. Each business has some control over price due to product differentiation but faces competition from substitutable products. Key features include differentiated but substitutable products, many sellers and buyers, free entry and exit, and profit maximization through product differentiation and non-price competition like advertising. In long run equilibrium, firms earn only normal profits as entry by new firms eliminates excess profits. Output is lower and prices higher under monopolistic competition compared to perfect competition.