To help them to make sense of industries in which firms are interacting, economists group industries into three basic structures. These three structures are as follows:
Perfect competition happens in an industry when numerous small firms compete against each other. Firms in a competitive industry produce the socially optimal output level at the minimum possible cost per unit.
A monopoly is a firm that has no competitors in its industry. It reduces output to drive up prices and increase profits. By doing so, it produces less than the socially optimal output level and produces at higher costs than competitive firms.
An oligopoly is an industry with only a few firms. If they collude, they reduce output and drive up profits the way a monopoly does. However, because of strong incentives to cheat on collusive agreements, oligopoly firms often end up competing against each other.