WHAT DO POPULATIONS AND SAMPLES TELL US?
Ideally, an investigator would prefer to get the characteristics of a whole population, but most times this is not possible. In order to make any generalisations about a population, a sample is used as a representative of the population. However, for any population there are many possible samples.
The population for a study of infant health might be all children born in the Australia from 1980 to 1989. The sample might be all babies born on 7th May in any of those years.