There are four main types of markets: perfect competition, monopolistic competition, oligopoly, and monopoly. Perfect competition is characterized by many firms producing the same standardized product and taking the market price as given. Monopolistic competition involves many sellers offering similar but differentiated products that are close but not perfect substitutes. Oligopoly is defined by only a few firms in an industry due to barriers to entry like economies of scale or government limits. Monopoly exists when there is only one seller of a unique product without close substitutes.