Monopolistic competition is a market structure with many firms selling similar but not identical products. Firms have some control over pricing due to product differentiation, but competition is still present from close substitutes. While firms face downward sloping demand curves, the existence of substitutes limits their monopoly power. Firms must use non-price factors like advertising, location, and quality to increase prices and profits in both the short and long run. In the long run, new entrants will join the market until economic profits are eliminated.