Final answer:
Perfect competition is a market with many firms selling identical products, monopolistic competition involves firms selling similar but differentiated products, oligopoly is when a few firms dominate the market, and monopoly is when there is only one firm with a unique product and barriers to entry.
Step-by-step explanation:
The market structure characterized by many firms selling an identical product, with many buyers and no entry restrictions, is called perfect competition. A market with many firms selling similar but slightly different products and competing against each other is known as monopolistic competition. A structure where only a few firms exist and dominate the market, often selling identical or similar products, is termed an oligopoly. Finally, when a single firm produces a unique good or service with no close substitutes and is protected by barriers that prevent new firms from entering, the structure is known as a monopoly. In an oligopoly, firms may face the temptation to collaborate similarly to a monopoly or compete against each other, affecting market outcomes for prices and quantities. In monopolistic competition, firms seek to differentiate their products to stand out, which can lead to non-price competition, such as advertising, but also can result in inefficiencies as firms do not produce at the lowest point on their average cost curves.