Metropolitan Statistical Areas have at least one urbanized area of 50,000 or more population, plus adjacent territory that has a high degree of social and economic integration with the core as measured by commuting ties. They are defined in terms of whole counties (or equivalent entities). Nonmetro counties are outside the boundaries of metro areas and are further subdivided into two types: micropolitan areas and all remaining "noncore" counties.
The Metropolitan and Micropolitan Statistical Area Standards do not equate to an urban-rural classification; many counties included in Metropolitan and Micropolitan Statistical Areas, and many other counties, contain both urban and rural territory and populations.
Source and for further information: Office of Management and Budget http://www.whitehouse.gov/omb/fedreg/metroareas122700.pdf