This document defines key terms related to population and sampling: population is the total set of data, while a sample is a subset of the population. There are different random sampling techniques described, including simple random sampling by lottery, systematic random sampling by selecting every kth item, stratified random sampling by proportionally selecting from subgroups, and cluster sampling by randomly selecting whole groups. The document also distinguishes between probability sampling, which uses random selection, and non-probability sampling, which does not.