The U.S. Census Bureau defines an urban places as having a population density of at least 100 people per square mile and a total population of at least 2,500 people. All areas that do not meet those criteria are labeled as rural.
The Census Bureau's 2010 urban and rural definitions further classify urban into "urbanized areas" and "urban clusters" by using population counts and density. Urban clusters are census blocks with a population of 10,000 or more, and urban areas are blocks that contain 50,000 or more people. According to the 2010 census results, there are 486 urbanized areas and 3,087 urban clusters nationwide.
Prior to 2010, people could live in a very densely populated area, but if it was unincorporated and not part of an actual city, they were not classified as urban dwellers. The new categories account for the fact that many neighborhoods or suburbs surrounding a metropolitan area maintain a large enough population and great enough population density to be classified as an urban area. Rural populations, on the other hand, live in any area not defined as an urbanized area or urban cluster. Accordingly, a section of a metropolitan area or a cluster could be classified as rural even though it is technically within the boundaries of a major city.