Answer:
Urbanization: this is the process by which the population or urban areas increases, usually because people move out to the cities form rural areas.
Metropolitan area: an admnistrative divison consisting in a city, and its sorrounding areas.
Suburb: the area sorrounding a city that has a lower population density than the urban core, but a higher population density than a rural area.
The relationship between these three variables is very close. Urbanization is what drives the growth of both urban cores and suburbs, and a metropolitan area emerges when a city and its suburbs grow enough that they require a new special administrative division in order to coordinate local policies.
