Exurbs are residential communities that are connected to urban areas by function, but are separated by geography. Most people in an exurb work or are somehow dependent on a large urban city, but they are not connected by continuous residential settlement.
Exurbs come after the suburbs, both in time and distance. Suburbs were a product of the post-war period and the Baby Boom, but exurbs really came into their own after the completion of the Interstate Highway System, and the spread of big box sprawl. Exurbs across the United States are characterized by the same housing developments full of McMansions, endless parking lots full of big box stores, and highways and freeways that cut the town up. One of the ironies of exurbs is that people leave the cities to find peace and tranquility, and then ruin it for convenience.
But one of the real secrets of exurbs is in the name: urban and rural are both descriptions that, while ambiguous, denote something with a quality of its own. Rural areas were traditionally marked by the presence of agriculture or extractive industries. But an exurb is named simple as being something that is not urban. Exurbs have no characteristics of their own, and are seen merely as an addendum of satellite of an urban area.