An imperfect market is an environment in which all parties do not have complete information, and in which participants can influence prices. All markets are imperfect to some degree. Here are several examples of imperfect markets:
Monopolies and oligopolies. An organization could have established a monopoly, so it can charge prices that would normally be considered too high. The same situation arises in an oligopoly, where there are so few competitors that there is no point in competing on price.
State intervention. Governments may intervene in a market, usually to set prices below the actual market level (such as by subsidizing the price of oil). When this happens, an excessive quantity is purchased. The reverse situation can also occur, where a government imposes such high regulatory barriers that few companies are allowed to compete (see the preceding monopoly and oligopoly discussion).
Differing product features. An imperfect market can exist when competing products contain different features. When this is the case, buyers have a difficult time comparing the products, and so may pay too much for them.
The usual effect of an imperfect market is that astute traders take advantage of the situation. This may be monopoly owners who profit from excessively high prices, investors who buy or sell securities based on insider information, or buyers who engage in arbitrage to buy goods at artificially low prices and sell them elsewhere at higher prices.