The economic structure of a perfect world. We can accurately use the supply and demand curves to determine price, as all business's are the same. Every product is like the, and here no monopoly can occur. Competition is perfectly balanced and fair, and consumers will always buy products at equilibrium price. Any market in which a prices cannot be hampered expect by changes in supply or demand is a good example.
Competition is imperfect, but competition occurs. Different companies intend to control more of the market by offering different goods or services. Not all products are identical, and therefore consumer choice can move the price out of equilibrium. Most real life markets tend to copy this trend, and often for the better as it constantly encourages more unique or outright better products.
A form of monopolistic competition. Here very few corporate entities control a product. This almost always lead to an increase in price for the consumer as competition that would occur in a pure monopolistic system is much less important. Oligopolies however often much more responsive and informed on competition, both on the consumer and producer end. This allows faster, more accurate market decisions on both parties which can help prices move toward equilibrium like in a perfect market structure. Below you can see almost everything you watch in TV is control by one or two of three major entities.
The ultimate conclusion of monopolistic competition. As companies try to make better products for less, eventually one firm will produce a good that is better than any other product for less than any other product. This will lead to them alone being able to determine price, and other firms being unable to compete. Of all the monopolies, this one is the most efficient. Negative externalities are minimized, and resources are better allocated by a maximum efficiency entity. However if abuses in true competition are used to raise costs for competitors, than the firm will not achieve the efficiency as if they had competed fairly, and often result in higher prices. Power and electricity are often natural monopolies, due to having an established power grid makes them far more efficient than any new competitor could ever be.
A monopoly which is caused by location being an almost impossible to cross barrier of entry. Here one sole entity has access to a market, but is not necessarily the sole producer of a good. The sole advantage of a geographic monopoly is it allows that sole entity to profit from an otherwise unprofitable market if there was competition. The lack of competition however will naturally lead to an overpriced product. The British tried to create one with the Navigation Acts, by isolating the colonies of North America from Dutch and Spanish traders. This did ultimately push up prices, but this intern lead to increased competition as the law was defied.
A monopoly made by having a unique mean of production. Here a way a product is produced, the technology to produce it mostly, belongs to a sole firm and no other firms have access to it. This monopoly generally has a focus on increased productivity or quality, and is better for the consumers than other monopolies. However it is often achieved by stifling other technology and will always lead to higher prices. For example in the early years of the Polaroid picture. Its creator, Edwin Land, had sole access to methods in which instant cameras were produced and therefore no one else could access the technology to compete with him.
A monopoly caused by government intervention. Rather the government owning the means to control an goods production, or giving the privilege to a third private party, law forms an impassable barrier of entry for competition. For goods and services in which let a private entity control them would be dangerous, government control is a natural solution. Government monopolies however tend to be far less responsive to market trends, resulting in either overpriced or under-priced goods. For example, as Finland has deemed gambling dangerous to its population, all slot machines are produced and run by the government.