In the original sense, urbanization referred to the mass migration of people from the farming regions and rural estates to the industrial centers and commercial centers. The industrial revolution led people to new opportunities in life, and away from farming or servitude. The jobs that were available in manufacturing, and other areas of commerce were appealing to those who, for generations, had worked their fingers to the bone for a bare subsistence living.
The flood of people to the industrial and other job centers created population density, urban development, and the demand for government control and public services. Individuals found even more opportunity in providing services to the working masses. The industrial and job centers grew into large cities, became transportation, government and cultural hubs in their own rights, a nd siphoned off people from the major cities.
Urbanization seemed to be an endless proposition. In America, where there seemed to be unlimited land mass for well planned growth in housing, industrial centers, and service areas. But something happened. Industry began to flee the high priced cities and to establish production centers in outlying suburban areas. Working class city dwellers began to see the job base moving to outlying areas. The elderly who hoped to leave their city properties to the next generation were left behind in properties that were growing in value but also in cost and isolation.
But the cities were not abandoned. There was a new form of cross-urbanization instead. The working class was replaced by the well to do professional and business classes who could afford the increasingly costly lifestyle in the gentrified and refurbished city neighborhoods. The professional and business classes can migrate from anywhere and can migrate to anywhere. This can become volatile migration. Job markets, themselves can become portable, and thus volatile and fast moving.
Working class people now commute from the outling areas to the densely job rich urban job centers. The lowest classes find the urban areas enormously liveable, especially once the commuters leave vast downtown and business areas unoccupied for the night. The services that are established for substance abusers, the ultra poor, the mentally ill, and the chronically homeless attract them from all over the country.
As a result, the new form of urbanization in mature developed countries is causing a great divide between well to do and very poor residents that is filled during business hours by a transient commuter class, or on weekends by tourists and shoppers.
In developing countries, there is still the traditional urbanization, where farming is abandoned for opportunity in the cities. Those cities will have to go through the process of providing better public services, accepting a middle class and a working class, or they will deterioriate prematurely into impoverished and poorly controlled social disasters.