The population density of a country or city is a way of showing how crowded that place is. It is calculated by dividing the population by the area. For example, France has a population of 60,561,200, and an area of 551,695 square kilometres, so its population density is about 109.8 persons per square kilometre.
A number of factors can affect population density. For example, the climate. Greenland has a very low population density because it is very cold there, so not many people want to live there.
Many cities were built near rivers, because the first settlers wanted somewhere close to water to start a society, so you could say areas around rivers usually have a high population density.
Some places, such as cities, have very high population densities, so they are quite crowded. The population density of New York City is 10,292 persons per square kilometre. Other places, such as large countries, can have very low population densities. The population density of Canada is only 3.5 persons per square kilometre because it is such a big country.
The country with the highest population density in the world is Monaco, with 16,620 persons per square kilometre. The country with the lowest is Greenland, which has only 0.03 persons per square kilometre.