The black market is the trade of goods illegally. Black markets operate outside the normal economy. The black market is different from the so-called Grey market, where goods are traded legally, but outside the common channels. Goods and services traded on the black market include drug and arms trafficking as well as prostitution.
In some cases, the goods or services are cheaper than on the normal market, because taxes or production costs do not need to be paid - very often the goods traded were stolen. In other cases, the price on the black market is higher, because the goods are difficult to get, and producing or handling them may be dangerous. Prices in the illegal drug trade are often much higher than for similar but legitimate goods, for these reasons.