Monopolistic competition is characterized by many sellers offering differentiated products, free entry and exit into the market, and firms competing through non-price factors like advertising. In the short run, firms will earn economic profits or losses depending on demand. In the long run, free entry and exit will drive profits to zero as firms enter industries with profits and exit those with losses. While prices exceed marginal costs as in a monopoly, free entry ensures prices equal average costs in the long run as in perfect competition. However, monopolistic competition is not as efficient as perfect competition due to deadweight loss from prices above marginal costs.