Product differentiation under monopolistic competition refers to firms attempting to distinguish their products or services from their competitors. Differentiation can be achieved through branding, design, quality, features, customer service, location, or any other attributes that make a product appear unique to consumers.

The effects of product differentiation are significant and multifaceted. Firstly, it gives firms market power, enabling them to set prices above marginal cost and earn profits in the short run. Secondly, product differentiation leads to non-price competition. Firms invest in advertising, marketing, and innovation to enhance their product's appeal rather than competing solely on price. This can lead to increased consumer choice and drive product quality and innovation improvements.

Other economic effects include brand loyalty, where successful product differentiation can lead to reduced price elasticity of demand for a firm's product. This means customers are less sensitive to price changes. As a result of product differentiation, each firm faces a downward-sloping demand curve, which is more elastic than that of a monopolist due to the presence of available substitutes in the market.

However, product differentiation can also result in inefficiencies. The emphasis on branding and marketing may lead to higher costs, often passed on to consumers through higher prices.

In summary, product differentiation under monopolistic competition creates a market dynamic where firms compete on factors other than price, fostering innovation and variety and leading to potential inefficiencies and higher consumer prices.

It explains why similar products can coexist at different price points and why firms invest heavily in branding and unique product attributes. While it can lead to inefficiencies compared to perfect competition, it also drives innovation and provides consumers with a wide array of choices.