Monoculture is the practice of growing one crop over a particular area, and has become increasingly prevalent all over the world as the modern age has rolled on. Monoculture stands in sharp distinction to polyculture, which has been the practice of mankind for most of recorded history, and which remains prevalent in many parts of the developing world.
Monoculture is particularly suited to temperate Western climates, but still carries inherent risks. Polyculture evolved to provide genetic variability amongst crops so that particular parasites and diseases would not be able to impact the whole crop. However, this meant the plants were often irregular and hence not fit for modern machines or markets, which demand similarity of product. This trade-off between hardiness and regularity has been decided in favour of the latter, and to compensate for the deficiency in the former pesticides were invented. Chemicals used in modern farming are not merely just another performance-enhancing device, but are a bare necessity; modern crops simply would not survive without them. This means scientists are locked in a biological arms race against evolving pests.
In much of the developing world, polyculture is practiced because it provides variety of diet (by having totally different crops in the same field) and hardiness, which is more useful on a family farm. Westerners, from colonial administrators to World Bank agronomists, have tried to export monoculture to the developing world, often with disastrous consequences. For instance, monoculture is unsuited to poor soils because multiple crops are needed to maintain a nutrient balance in the soil. Farming in the developing world may look chaotic and inefficient, but it is typically well attuned to local needs - modern science can certainly help it, but attempts to supplant traditional knowledge entirely are doomed to end in failure and famine.