This document discusses key concepts in sampling, including population, sample, target population, sampling unit, sampling frame, and different sampling schemes. It defines population as the set of all measurements of interest, and sample as a subset of the population. Probability and non-probability sampling methods are described. Probability sampling, such as simple random sampling, systematic sampling, stratified sampling, and cluster sampling allow results to be generalized to the population, while non-probability convenience and snowball sampling do not.