I explained this to someone the other day;
city - large settlement of 80,000 people or more. often contains major transport links (airports, motorways)
town - settlement of anything from 10,000 - 70,000 people. Often connected with rail and road to cities and villages, used by people who work in cities as a place to live.
suburb - the areas of housing that surround a city, or town that are used exclusively for houses.
village - a small settlement of less than 10,000 people. Often poorly-connected with very few facilities (shops, stations, etc.). Situated in the countryside.
countryside - any area that is not within the boundaries of a town or city.