Rural areas have low population densities and consist mostly of undeveloped land used for farming. Urban areas have higher population densities, more buildings and infrastructure like roads and railways, and land is used more for commercial and industrial activities. Rural-urban migration is the movement of people from rural to urban areas due to "push" factors like lack of jobs in rural areas and "pull" factors like availability of jobs and amenities in cities. This migration causes problems like overcrowding and pollution in cities as well as rural depopulation as working-age people leave for opportunities elsewhere.