The document summarizes key concepts relating to firms operating in competitive markets including: - The four basic market types are perfect competition, monopolistic competition, oligopoly, and monopoly. - For a competitive firm, marginal revenue equals price since each additional unit sold does not impact the market price. - A competitive firm will produce the quantity where marginal revenue equals marginal cost to maximize profits. - In the short-run, a competitive firm will shut down if price falls below average variable cost. In the long-run, the firm will exit the market if price falls below average total cost.