Population: refers to the population of the target you want to count.

Individual: refers to one of the whole group.

Sample: extract some units from the population and estimate the characteristics of the population through statistical samples.

Sample size: it is the number of samples you take.

For example, if we want to count the ratio of male to female in senior three, then all the students in senior three are "whole" and each student is "individual". You randomly select 200 people for statistics. These 200 people are "samples" and 200 are "sample size".