Most of the counties in the US lie outside of metropolitan areas (roughly two-thirds). These nonmetro counties are typically thought of as rural by most people. But all rural areas are not the same–many have an urban core, albeit a small one, which dominates a central county and to which people in surrounding counties may commute. These areas have recently been designated “micropolitan areas” by OMB to differentiate them from other nonmetro areas. Here’s the Economic Research Service on the basic definition of micropolitan areas:

Office of Management and Budget (OMB) was urged by various sources in the last decade to delineate the entire land surface of the country into areas, and not leave the territory outside of metro areas as an undifferentiated residual. As a partial response, OMB designated micro areas using the same procedure as that for metro areas. Any nonmetro county with an urban cluster of at least 10,000 persons or more becomes the central county of a micro area. As with metro areas, outlying counties are included if commuting to the central county is 25 percent or higher, or if 25 percent of the employment in the outlying county is made up of commuters from the central county. Because they are county-based and include outlying areas, the total area population reaches well beyond 50,000 for many micro areas. The inaugural set of 560 micro areas includes 674 counties and ranges in size from 13,000 (Andrews, Texas) to 182,000 (Torrington, Connecticut).
Micro areas contain just under 60 percent of the nonmetro population, with an average of 43,000 people per county. In contrast, the 1,378 “noncore” counties, with no urban cluster of 10,000 or more residents, average just 14,000 people.

It is these micropolitan areas that include the bulk of the rural population, as mentioned above, even though they are only about a third of rural counties and include only about a quarter of rural land area. They tend to be more economically dynamic and, of course, are far denser than other rural counties, with about a third of micropolitan residents actually living in the principal cities of these areas (the corresponding figure for metro areas is not so much higher, about 40 percent).
Much more fascinating detail on micropolitan areas, as well as the entire new system of metro classification promulgated by OMB, may be found in this excellent report from the Brookings Institution, “Tracking Metropolitan America into the 21st Century: A Field Guide to the New Metropolitan and Micropolitan Areas“.
Are their political implications to this? Sure. If Democrats want to improve their performance in rural areas, that must principally be about improving their performance in micropolitan areas. Three-fifths of the rural vote and three-fifths of Bush’s gains in rural areas in 2004 were in micropolitan areas. And micropolitan areas, by virtue of their relatively high density and significant urban populations, should be more cosmopolitan and more open to Democratic appeals than the thinly-populated, non-micropolitan areas that make up the rest of rural America.