A population is every unit you could ever measure. A sample is the handful you actually did. That's a difference in KIND, not just size — and it's exactly why sample standard deviation needs a correction that population standard deviation never does.
A population is the complete set of everything you actually care about — every unit ever made, every measurement that could ever be taken. A sample is the smaller subset you actually measured or collected. That single distinction is why the sample standard deviation formula divides by (n−1)instead of n — a detail called Bessel's correction that trips up almost everyone who first meets it, because it looks like it should just be a typo. It isn't. It exists because a sample's own mean is, by construction, the single number that minimizes squared deviations within that specific sample — which means the sample slightly under-represents how spread out the real population actually is. Dividing by (n−1) instead of n corrects for that built-in bias.