Instead of enumerating all the sampling units in the selected clusters, one can obtain better and more efficient estimators by resorting to sub sampling within the clusters. The technique is called two-stage sampling, clusters being termed as primary units and the units within the clusters as secondary units.
The technique may be generalized to what is called multistage sampling. As the name suggests, multistage sampling refers to a sampling technique which is carried out in various stages. Here the population is regarded as made of a number of primary units, each of which is further composed of a number of secondary stage units and so on, till we ultimately reach the desired sampling unit in which we are interested.