Demography is the statistical study of human populations: their size, composition, and how they change through the interplay of fertility (births), mortality (deaths), and migration.
The Demography of the World Population from 1950 to 2100. Data source: United Nations — World Population Prospects 2017
Statistics is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, industrial, or social problem, it is conventional to begin with a statistical population or a statistical model to be studied. Populations can be diverse groups of people or objects such as "all people living in a country" or "every atom composing a crystal". Statistics deals with every aspect of data, including the planning of data collection in terms of the design of surveys and experiments.
Bernoulli's Ars Conjectandi was the first work that dealt with probability theory as currently understood.
Carl Friedrich Gauss made major contributions to probabilistic methods leading to statistics.
Karl Pearson, a founder of mathematical statistics