What is the difference between a population and a sample?
A population is the set of all possible units/observations (of interest) that could be selected/observed.
A sample is a (small) portion of a population.
Quite often in statistics, gathering data from a whole population is not time- or cost-effective. It would take too much time to ask every eligible voter in our country who they will vote for, and we certainly would not want to break every 2-by-4 made at a lumber yard in order to test their breaking strength.
This is why we use a sample. If selected properly, a sample can be a very good substitute for the population it's from, meaning we can use the sample to infer information about the population.