Other non-radio types of people dont easily grasp the notion of a market. Its not just a city, its a collection of cities, towns, townships, mudflats and urban sprawl around a major metropolitain area.
Some portions of America are more inportant to a radio man than others. and some do not even exist at all. (in the tree falls in the woods sense) These are called "un-rated market areas." Wyoming contains a whole lot of un-rated market area. Thsi is beacuse its population density hovers around 2 per square mile. or about 400,000 people. i.e. less than the City of Providence Rhode island.
This handy-dandy map color codes everything a radio-man should care about:
The kind People at Arbitron study this so that I dont have to.
The list is a tad out of date. but you can get a hard biannually updated list here: