Monopoly: There is a single seller in the market
Oligopoly: every company in this market structure is aware of the actions of the other companies (oligopolies are a small number of companies controlling the markets- there are elements of collusion in this structure because the firms work together to control prices and the market)
Perfect Competition: There are no barriers to entry (lots and lots of competing companies that each have a small share of the market)
Collusion: 3 companies secretly enter into a price agreement (this is illegal in many cases)
