what is a monopoly?

A monopoly is a type of market structure where one firm dominates the market for a specific good. An example of a monopoly is Google, as they essentially own the market of search engines. While other competitors do exist, such as Microsoft or Yahoo, their market share is too small to compete. 
Main characteristics: Single seller - a single firm is producing a good or a service for the entire market and it owns a very large share of the market Lack of close substitutes - there are no suitable alternatives to the product produced by the monopoly, leaving the consumer with only one choice Significant barriers to entry - other firms are not able to enter the monopoly market (reasons for this could be because the existing firms owns such a large portion of the market, because there are legal barriers such as patents or licenses or because the existing firm has complete control over the resources) 

A monopoly has a relatively inelastic demand curve meaning that demand is not very responsive to a change in price (since this good is the only option in the market leaving consumer with no choice but to buy this one good).
A monopoly will choose to produce at the level MC = MR which is the profit maximising level of output (Q on the graph).
The price that the monopoly is charging (Pm) is therefore higher than the price they are paying to produce (Pe which corresponds to the ATC, Average Total Cost, line).
Since revenue > costs the monopoly is making profit A monopoly will always aim to make a profit leaving a lot of room for discussion of whether it is an efficient market structure or not.