Final answer:
In pure competition, the short run is distinguished by fixed inputs and the inability of firms to enter or exit the market, while the long run allows full operational adjustments, including firm entry and exit, driving market price to a point of zero economic profit.
Step-by-step explanation:
The distinction between the short run and the long run in pure competition, like in a perfectly competitive market, primarily revolves around the ability of firms to enter and exit the market. In the short run, firms face fixed inputs and can't readily alter the scale of their operations. Firms aim for the output where profits are maximized or losses are minimized, producing at the level where P = MR = MC. However, they are unable to exit the market due to fixed costs and commitments.
In contrast, the long run is characterized by the absence of fixed inputs and the ability for firms to fully adjust their operations, including the entry of new firms and the exit of existing ones, in response to economic profits or losses. This adjusts the supply curve and leads the market back to a point where firms earn zero economic profits, where P = MR = MC and P = AC, at the minimum of the AC curve.