In common usage, a population is a distinct group of people with a common citizenship, identity, or characteristics.
In statistics, a population is a representative sample of a larger group of people (or even things) with one or more common characteristics.
Sample members must be randomly selected so that the results of the study accurately reflect the whole.
The US Census is perhaps the most ambitious survey in existence, given that it involves a door-to-door survey of the entire population rather than a selective group survey.
Population surveys, large and small, provide information for many if not most government and business decisions.