In a general sense we've seen that we can view most industries as characterized by one of four main industrial structures. These are:

Perfect Competition:

Many small firms producing identical products in an industry with no important entry barriers.

Monopoly:

A single firm in an industry with extremely high barriers to entry.

Monopolistic Competition:

Many small firms producing differentiated products in an industry with no important entry barriers.

Oligopoly:

A few firms in an industry with significant barriers to entry whose every decision must be based on strategic considerations of competitors' response.