More people are living in cities due to a phenomenon called urbanization, where the proportion of the population residing in urban areas increases over time. Cities have large populations sizes due to natural increases in births over deaths as well as rural-urban migration as people move from rural to urban areas to live and work. Cities are characterized by high population densities, large built up areas with dense transportation networks, tall buildings, and amenities like schools and malls, though they also face challenges like housing shortages and traffic congestion.