How is monopoly similar or different from monopolistic competition and from oligopoly?

A monopoly is a market structure where a single firm sells a dominant share of the quantity in a market. They occur due to geography, low-cost production, regulation or technological secrets, patents, or copyrights. Firms who hold the monopoly are protected by significant barriers to entry and exit. The market price in a monopoly is higher and the quantity sold is lower than a competitive market.

Monopolistic competition is a market structure where many firms sell similar yet, slightly different products. The market is not protected by significant barriers to entry so there is major price competition. Because there are many firms and the new firms can join the market, this is a major difference from monopolies.

An oligopoly is a market structure where a few large firms sell similar products, which is another difference from a monopoly. Unlike a monopoly, a firm in an oligopoly does not have any pricing power because if they increase their price, consumers will not buy their products since consumers will buy from a competitor. But similar to a monopoly, oligopolies are protected by significant barriers to entry and exit.