Population density is the measure of the population number per unit area, according to About.com. An example would be people per square mile, which is calculated by dividing the total number of people by the land area in square miles.
Population density is often used to estimate the number of people in any given area of a country. Examples include the population density of France, which is France's population divided by the square number of kilometers, which is approximately 109.8 people per square kilometer.
The Earth's population density is equal to its population divided by total square miles. The population is equal to 7 billion, and the square miles are 197 million including land and water. These two figures can be divided to come up with the population density of 35 people per square mile.
City-states, microstates and dependencies tend to have a much higher population density because the areas are very small but are inhabited by a large number of people. Some of these areas are considered to be overpopulated, though this depends on factors such as housing quality and access to resources. The most dense of populations are generally located in countries in eastern Asia and northern Africa.