Population density is the number of persons living within a unit of land area. The number of people (on average) living on a square mile of land would be a measure of population density. It is most frequently calculated by dividing the population of a specific geographic area (such as a county) by the surface area.
The map above shows the population density per square mile for counties in West Virginia. It is one of a large number of interesting maps that can be found in the West Virginia Health Status Atlas, a website maintained by the West Virginia Department of Health and Human Resources.
The West Virginia map above uses counties as enumeration units. However the world map above has dots position in relation to the number of people in a smaller area. It gives a more accurate representation of the distribution of people rather than an average by county or country. For a closer look see this world population density map enlarged.