The extremes in market structures are perfect competition on the one hand, and monopoly on the other. In perfectly competitive markets , there are many sellers with identical products. There are no barriers to entry or exit. To be truly perfectly competitive, there should be no transactions costs and all market participants should have complete information for making optimal choices. In perfect competition, price is driven to the marginal cost of production as the market finds an equilibrium along the market supply curve.
Monopoly is at the other end of the spectrum.
The monopolist is protected from competition by some kind of barrier to entry. Supply and demand analysis is standard for perfect competition, but does not work for modeling monopoly, because the monopolist has no supply curve. The model of monopoly behavior is not entirely different from perfect competition, however, because both stem from the profit maximization hypothesis and both conclude that firms will set marginal revenue equal to marginal cost. One key difference is that firms in perfect competition will be price takers whereas monopolists are price setters. Since the monopolist is a price setter facing the whole market demand curve, marginal revenue will be lower than price.