A city is generally speaking a very densely populated area, usually consisting of residential, industrial and business areas. A large percentage of a city is generally taken up by buildings, streets and parks. Waterways and lakes are usually the only undeveloped areas within a city center.
The geographies of cities are diverse. Often cities will be either coastal and have a harbor or they will be situated in close proximity to a river. This is mainly for historical reasons, as river and ocean transport in bygone days were generally cheaper and more efficient than road transport.
Older European cities which have not been extensively rebuilt tend to have city centers where the street are jumbled together, often seemingly without a structural plan. This is a hangover from a time before city planning and is usually perceived by tourists to be quaint and picturesque. Modern city planning has seen many different schemes for how a city should look. The most commonly seen pattern is the grid, almost a rule in the United States, and used for hundreds of years in China.
During the European middle ages, a city was as much a political entity as a collection of houses. A city could often have its own legislature, and sometimes a city could be directly under the supervision of the king, circumventing local noblemen.
The Industrial Revolution led to the rise of the modern great cities. Prior to that, cities were trading centers, but their populations were in general relatively small. There were exceptions such as the ancient cities of Rome and Byzantium, and 17th-century London. With the Industrial Revolution, as national economies changed from agrarian to industrial, huge numbers of people migrated from rural communities into the cities.
Modern cities are known for creating their own microclimates. This is due to the large clustering of hard sufaces that heat up in sunlight and that channel rainwater into underground ducts. As a result, city weather is often windier and cloudier than the weather in the surrounding countryside. Conversely, because these effects make cities warmer than the surrounding area, tornados tend to go around cities. Additionally cities can cause significant downstream weather effects.