Farm land is often included in the definition of undeveloped land. Under this definition, land is developed for urban uses like houses, businesses and industrial facilities. Urban development usually requires services like municipal water and sewer systems in order to support larger populations. Farm land isn't usually equipped with urban services, largely because fewer people live there.
A considerable amount of undeveloped land is held by the government. This land is not equipped with the urban services needed to support houses and businesses. The government--through agencies like the Bureau of Land Management--conserves this land for the economic value of timber and pasture.
Other undeveloped land is set aside in its natural state. A stricter definition of undeveloped land may include only these areas. This land is managed to conserve recreational and ecological value, but is not usually used for its natural resources. Legislation like the Wilderness Act and National Park Service Organic Act sets aside this land for perpetual protection.