A city is a large human settlement usually with dense population and majority of its residents are occupied in non-rural and non-agricultural pursuits. A common way to distinguish a city from other human settlements is by its population size, its functions and symbolic status.
Cities where formerly categorized in five grops such as-
1. Third world cities
2. Cities in socialists countries
3. Older (former)industrial cities
4. Global or world cities-
5. New industrial cities

Read more on classification of cities Here-http://www.jotscroll.com/forums/3/posts/166/city-definition-types-classification-and-characteristics.html