What are the different kinds of monopolies and how do they differ?

A natural monopoly is one that forms because the market has such high startup or fixed costs that no new firms are able to enter the market and compete with the existing firm because the incumbent firm is able to take advantage of significant economies of scale. A legal monopoly is one that is formed because it has some sort of protection from the government that acts as a barrier to entry. Examples of these protections include patents and copyrights. Finally, a government monopoly is one that is formed and given specific permission by the government to be the only seller in the market. An example of a government monopoly is the United States Postal Service.