A population is the set of all the individuals of interest in a particular study. Populations can obviously vary in size from extremely large to very small, depending on how the researcher defines the population. The population being studied should always be identified by the researcher.
A sample is a set of individuals selected from a population, usually intended to represent the population in a research study. a researcher studies a sample and then generalizes the results from the sample to the population.
Gravetter, F. J. & Wallnau, L. B. (2013). Statistics for the behavioral sciences (9th ed.). Belmont, CA: Wadsworth.