An urban area is an area where many people live and work close together. The population density is higher than in the surrounding area. It is where buildings are close together. Urban is the opposite of rural, where farm lands and nature are. Urban areas are usually cities and towns. Most of the work available in urban areas is factory and office work. agricultural work is rare because buildings are close together and there's no space for farm lands.
There are different urban area definitions for each country. The definitions mostly depend on how populated the country is and if the definition would show a true percentage of how urbanized the country is. For example, in Poland an urban area is any place that has the status of a town, whereas in China it's any district, city, or town with a population density higher than 1500 people per square kilometer. Urban population definition for Canada, Scandinavian contries is when people's houses are at least every 200 meters and if you have a house every 200 meters it is classified as urban.
A large urban area, with all connected areas (usually developed around some city) is called Metropolitan area.