This document provides an overview of different market structures: pure competition, pure monopoly, monopolistic competition, and oligopoly. It defines each structure and discusses their key characteristics. Pure competition is characterized by many small sellers, homogeneous products, perfect information and mobility. A pure monopoly has a single seller, large barriers to entry, and wields substantial influence over prices. Monopolistic competition involves many sellers of differentiated products. Oligopoly is dominated by a small number of interdependent firms. The document also outlines the assumptions of each market structure model and provides examples.