DEFINITIONS
Population
A population is any entire group with at least one characteristic in common. A survey covering the a population is commonly called a census. In a census every member of the group is included and the entire group is used to characterise the population.
Sample
A sample is part of a population. It is a subset of the population, often randomly selected for the purpose of studying the characteristics of the entire population.
Estimate
An estimate is information about a population extrapolated from a sample of the population.