A city is a relatively large and permanent settlement. Although there is no agreement on how a city is distinguished from a town within general English language meanings, many cities have a particular administrative, legal, or historical status based on local law.
Cities generally have complex systems for sanitation, utilities, land usage, housing, and transportation. The concentration of development greatly facilitates interaction between people and businesses, benefiting both parties in the process. A big city or metropolis usually has associated suburbs and exurbs. Such cities are usually associated with metropolitan areas and urban areas, creating numerous business commuters traveling to urban centers for employment. Once a city expands far enough to reach another city, this region can be deemed a conurbation or megalopolis.
History
Towns and cities have a long history, although opinions vary on whether any particular ancient settlement can be considered a city. A city formed as central places of trade for the benefit of the members living in close proximity to others facilitates interaction of all kinds. These interactions generate both positive and negative externalities between others’ actions. Benefits include reduced transport costs, exchange of ideas, sharing of natural resources, large local markets, and later in their development, amenities such as running water and sewage disposal. Possible costs would include higher rate of crime, higher mortality rates, higher cost of living, worse pollution, traffic and high commuting times. Cities grow when the benefits of proximity between people and firms are higher than the cost.
Source: Wikipedia