Products can be identical or differentiated. The key is that there are few enough producers that each one has a fairly large part of the market. Large enough that any individual producer can affect what happens in the market. Because everyone`s actions matter. The producers become mutually interdependent. Whatever one does, affect everyone else. This mutually interdependence actually makes olipoly the most complicated type of market structure to operate in. One of the fiercest oligopolistic competitions exists in the automobile industry across the whole world.
In the end I can sum up that there are some situations in market when the monopoly is just necessary for the government and state. But the most common types of market are oligopoly and monopolistic market structures, because they don`t have so strict barriers to entry and everyone can easily operate in such market because of the regulations.
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