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Etymologically, demography comes from the Greek words demos (for population) and graphia (for description or writing).Demography stated informally tries to answer the following questions:
– How many people of what kind are where?
– How did the number of people come about?
– What is the implication of the number derived?
Formally, demography is the scientific study of human population and its dynamics.
Demography deals with aggregates of individuals, it describes the characteristics of population. Most demographic studies employ quantitative and statistical methods, features of population are often measured by counting people in the whole population or sub-populations and comparing the counts.
Population size is a number with absolute and relative connotations. In the absolute sense, human population size quantifies the number of people in a country, region or space. Beyond the numerical quantity is the concern for distribution both within and among country, region, or space, this accounts for the relative connotation. Resulting from the concepts of population size and distribution is population density which is the relationship between population size, distribution, and the space that contains it.
Population density is consequential to the well being of the population. Notably, population density explains the viral spread of disease, knowledge, and ideas; epidemics is most likely to occur in a densely populated space as knowledge and ideas can easily diffuse.
Population study is concerned with the size and distribution of identifiable subgroups within populations. This concern yields information on the structure and composition of population. The characterization (categorization or classification) of population relies on endless list of traits- age, gender, education, religion, income, occupation, language, race, ethnicity etc. However, some traits are more useful; traits that change less frequently or has predictable pattern of change. Age and gender are the basic and most influential characteristics to demographic processes, hence they are known as demographic characteristics.
The dynamics of population is rooted in the basic demographic processes of birth, death, and migration. Basically, population changes can be associated with leaving or entering; to leave means dying or emigrating and to enter means being born or immigrating. This fact can be depicted in the basic demographic equation that follows:
Pt+1 = Pt + Bt ,t +1 – Dt ,t +1 + It ,t+1 – Et ,t+1
where Pt is the number of persons at time t and the number of persons one year later is Pt ,t+1; Bt ,t+1 and Dt ,t+1 are the number of births and deaths that occur between times t and t+1 respectively; It ,t+1 and Et ,t+1 represent the number of immigrants to and emigrants from the population respectively between times t and t+1.
The difference between Bt ,t+1 and Dt ,t+1 is referred to as natural increase (or decrease when the difference is negative) while the difference between It ,t+1 and Et ,t+1 is known as positive net international migration when the difference is positive and negative net international migration otherwise.
Growth in demographic parlance refers to change in population size. From the demographic equation above, growth means the difference between Pt+1 and Pt even though this difference is negative. The interplay of demographic processes results in population growth as well as compositional changes in population.

Readings
David Yaukey and Douglas L. Anderton, Demography: The Study of Human Population 2nd ed., 2001
Dudley L. Poston, JR. and Leon F. Bouvier, Population and Society: An Introduction to Demography, 2010