Population is the whole dataset.

The population contains every possible element that satisfies a specific predicate (condition).

Sample is a subset/slice of the population, it is often randomly picked. A census is used to collect information about the population.

Census vs. sample

A census provides an entirely accurate result, however is time consuming, expensive, and cannot be used if the item is destroyed.

A sample is faster, requires fewer people/items, and therefore has less data to process, however maty not be as accurate as a census and may not be large enough to provide certain information.