Perfect competition is an economic model that describes a hypothetical market form in which no producer or consumer has the market power to influence prices. According to the standard economical definition of efficiency (Pareto efficiency), perfect competition would lead to a completely efficient outcome. The analysis of perfectly competitive markets provides the foundation of the theory of supply and demand.
To be exhaustive, note that some economists do not agree with this presentation of the model of perfect competition. Many reasons are advanced, but one of the main is that it focuses on unnecessary conditions (atomicity, perfect information...) while it does not allow an answer to the question : "If agents are price-takers, who sets the prices ?" Indeed, in this model, as firms and consumers can not set the prices, it can't be—as it is often said (e.g. below)—that it is the firms who fix them. So, actually, there is a need for a benevolent agent who proposes prices to firms and consumers and fixes the ones at which exchange will occur. They also think that the argument that a global entity called "the market" could fix the prices, when its constituents (producers and consumers) can not is greatly disturbing. Above other criticism, there is also the lack of emphasis on the fact that no uncertainty about future prices or incomes, no transport cost, and no indivisibility can be integrated in this model.
In contrast to a monopoly or oligopoly, it is impossible for a firm in perfect competition to earn abnormal profit in the long run, which is to say that a firm cannot make any more money than is necessary to cover its economic costs. If a firm is earning abnormal profit in the short term, this will act as a trigger for other firms to enter the market. They will compete with the first firm, driving the market price down until all firms are earning normal profit, it could be said that abnormal profit is 'competed away'. On the other hand, if firms are making a loss, then some firms will leave the industry, reduce the supply and increase the price. Therefore, all firms can only make normal profit in the long run.
It is important to note that perfect competition is a sufficient condition for allocative and productive efficiency, but it is not a necessary condition. Laboratory experiments in which participants have significant price setting power and little or no information about their counterparts consistently produce efficient results given the proper trading institutions (Smith, 1987, p. 245).