When a researcher divides the entire population into sections, it is called

Dividing the entire population into sections or groups and then sampling whole groups is cluster sampling.

Clusters are natural groupings such as schools, blocks, or villages taken as units.

Stratified sampling instead divides the population into homogeneous strata and draws from every stratum.

Convenient sampling picks whoever is readily available, with no division of the population.

Systematic sampling selects every kth element from an ordered list.

Cluster sampling lowers cost and effort but usually raises sampling error compared with stratified sampling.