A monopoly is a market dominated by one single firm that manufactured the entire amount that is supplied of a certain product. This firm has a huge bargaining power and is able to fix the market price of its products, consumers can either accept and pay such price or refuse to purchase it. Therefore, monopolies face a very steep and inelastic demand curve.
Monopolistic competition is a market structure constituted by a large number of different firms that supply a product that is essentially the same (products that have certain differences but can be used to satisfy the same need). These firms are not price-takers as they have the power to differentiate their products, so that consumers perceive them in a different and superior manner than the products of the competitors, and hence they would be willing to pay if a higher price is fixed.
In both market structures, the firm has a certain degree of control over the price of the product (although the power is absolute in the case of the monopoly and much sligther in the case of monopolistic competition). Therefore, the main difference is the number of firms operating on each type of market.
