Urban areas are formed by urbanization and are classified as cities, towns, conurbations, or suburbs by urban morphology. The phrase contrasts with rural regions such as villages and hamlets in urbanism; it contrasts with the natural environment in urban sociology or urban anthropology. An urban area can be defined by its population density as a number of people per square kilometer (km2). There are very high-density cities with over 10,000 people per km2, mainly in Asia and Africa; mid-range cities with between 500 and 10,000 people per km2, for example London, New York City, Chicago, Madrid, and Sydney; and low-density cities with below 500 people per km2, such as Bangkok, Lagos, and Mexico City. The population of an urban region may be highly diverse, with many different cultures and ethnic groups living side by side. In large cities in developed countries, there is a tendency for immigrants to cluster together in specific areas; these are known as ghettoes or barrios. In less developed countries, urban centers tend to be more homogeneous with a single culture within their boundaries. In general, cities are characterized by modern industry and technology, a centralized government system, permanent residential buildings, and a large population. However, there are other factors that can make places worthy of being called cities.