Muhammad Touhidul Islam explains what cluster sampling is and why it can be useful:
Cluster sampling is defined as a sampling method where multiple clusters of people are created from a population where they are indicative of homogenous characteristics and have an equal chance of being a part of the sample. In this sampling method, a simple random sample is created from the different clusters in the population. This is a probability sampling procedure.
Click through for a few examples of where this can be useful.